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en/codes/csharp/chapter_sorting/bubble_sort.cs

@@ -0,0 +1,51 @@
+/**
+ * File: bubble_sort.cs
+ * Created Time: 2022-12-23
+ * Author: haptear (haptear@hotmail.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class bubble_sort {
+    /* Bubble sort */
+    void BubbleSort(int[] nums) {
+        // Outer loop: unsorted range is [0, i]
+        for (int i = nums.Length - 1; i > 0; i--) {
+            // Inner loop: swap the largest element in the unsorted range [0, i] to the rightmost end of that range
+            for (int j = 0; j < i; j++) {
+                if (nums[j] > nums[j + 1]) {
+                    // Swap nums[j] and nums[j + 1]
+                    (nums[j + 1], nums[j]) = (nums[j], nums[j + 1]);
+                }
+            }
+        }
+    }
+
+    /* Bubble sort (flag optimization) */
+    void BubbleSortWithFlag(int[] nums) {
+        // Outer loop: unsorted range is [0, i]
+        for (int i = nums.Length - 1; i > 0; i--) {
+            bool flag = false; // Initialize flag
+            // Inner loop: swap the largest element in the unsorted range [0, i] to the rightmost end of that range
+            for (int j = 0; j < i; j++) {
+                if (nums[j] > nums[j + 1]) {
+                    // Swap nums[j] and nums[j + 1]
+                    (nums[j + 1], nums[j]) = (nums[j], nums[j + 1]);
+                    flag = true;  // Record element swap
+                }
+            }
+            if (!flag) break;     // No elements were swapped in this round of "bubbling", exit directly
+        }
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = [4, 1, 3, 1, 5, 2];
+        BubbleSort(nums);
+        Console.WriteLine("After bubble sort, nums = " + string.Join(",", nums));
+
+        int[] nums1 = [4, 1, 3, 1, 5, 2];
+        BubbleSortWithFlag(nums1);
+        Console.WriteLine("After bubble sort completes, nums1 = " + string.Join(",", nums1));
+    }
+}

en/codes/csharp/chapter_sorting/bucket_sort.cs

@@ -0,0 +1,46 @@
+/**
+ * File: bucket_sort.cs
+ * Created Time: 2023-04-13
+ * Author: hpstory (hpstory1024@163.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class bucket_sort {
+    /* Bucket sort */
+    void BucketSort(float[] nums) {
+        // Initialize k = n/2 buckets, expected to allocate 2 elements per bucket
+        int k = nums.Length / 2;
+        List<List<float>> buckets = [];
+        for (int i = 0; i < k; i++) {
+            buckets.Add([]);
+        }
+        // 1. Distribute array elements into various buckets
+        foreach (float num in nums) {
+            // Input data range is [0, 1), use num * k to map to index range [0, k-1]
+            int i = (int)(num * k);
+            // Add num to bucket i
+            buckets[i].Add(num);
+        }
+        // 2. Sort each bucket
+        foreach (List<float> bucket in buckets) {
+            // Use built-in sorting function, can also replace with other sorting algorithms
+            bucket.Sort();
+        }
+        // 3. Traverse buckets to merge results
+        int j = 0;
+        foreach (List<float> bucket in buckets) {
+            foreach (float num in bucket) {
+                nums[j++] = num;
+            }
+        }
+    }
+
+    [Test]
+    public void Test() {
+        // Assume input data is floating point, interval [0, 1)
+        float[] nums = [0.49f, 0.96f, 0.82f, 0.09f, 0.57f, 0.43f, 0.91f, 0.75f, 0.15f, 0.37f];
+        BucketSort(nums);
+        Console.WriteLine("After bucket sort completes, nums = " + string.Join(" ", nums));
+    }
+}

en/codes/csharp/chapter_sorting/counting_sort.cs

@@ -0,0 +1,77 @@
+/**
+ * File: counting_sort.cs
+ * Created Time: 2023-04-13
+ * Author: hpstory (hpstory1024@163.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class counting_sort {
+    /* Counting sort */
+    // Simple implementation, cannot be used for sorting objects
+    void CountingSortNaive(int[] nums) {
+        // 1. Count the maximum element m in the array
+        int m = 0;
+        foreach (int num in nums) {
+            m = Math.Max(m, num);
+        }
+        // 2. Count the occurrence of each number
+        // counter[num] represents the occurrence of num
+        int[] counter = new int[m + 1];
+        foreach (int num in nums) {
+            counter[num]++;
+        }
+        // 3. Traverse counter, filling each element back into the original array nums
+        int i = 0;
+        for (int num = 0; num < m + 1; num++) {
+            for (int j = 0; j < counter[num]; j++, i++) {
+                nums[i] = num;
+            }
+        }
+    }
+
+    /* Counting sort */
+    // Complete implementation, can sort objects and is a stable sort
+    void CountingSort(int[] nums) {
+        // 1. Count the maximum element m in the array
+        int m = 0;
+        foreach (int num in nums) {
+            m = Math.Max(m, num);
+        }
+        // 2. Count the occurrence of each number
+        // counter[num] represents the occurrence of num
+        int[] counter = new int[m + 1];
+        foreach (int num in nums) {
+            counter[num]++;
+        }
+        // 3. Calculate the prefix sum of counter, converting "occurrence count" to "tail index"
+        // counter[num]-1 is the last index where num appears in res
+        for (int i = 0; i < m; i++) {
+            counter[i + 1] += counter[i];
+        }
+        // 4. Traverse nums in reverse order, placing each element into the result array res
+        // Initialize the array res to record results
+        int n = nums.Length;
+        int[] res = new int[n];
+        for (int i = n - 1; i >= 0; i--) {
+            int num = nums[i];
+            res[counter[num] - 1] = num; // Place num at the corresponding index
+            counter[num]--; // Decrement the prefix sum by 1, getting the next index to place num
+        }
+        // Use result array res to overwrite the original array nums
+        for (int i = 0; i < n; i++) {
+            nums[i] = res[i];
+        }
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = [1, 0, 1, 2, 0, 4, 0, 2, 2, 4];
+        CountingSortNaive(nums);
+        Console.WriteLine("After counting sort (cannot sort objects) completes, nums = " + string.Join(" ", nums));
+
+        int[] nums1 = [1, 0, 1, 2, 0, 4, 0, 2, 2, 4];
+        CountingSort(nums1);
+        Console.WriteLine("After counting sort completes, nums1 = " + string.Join(" ", nums));
+    }
+}

en/codes/csharp/chapter_sorting/heap_sort.cs

@@ -0,0 +1,52 @@
+/**
+* File: heap_sort.cs
+* Created Time: 2023-06-01
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_sorting;
+
+public class heap_sort {
+    /* Heap length is n, start heapifying node i, from top to bottom */
+    void SiftDown(int[] nums, int n, int i) {
+        while (true) {
+            // If node i is largest or indices l, r are out of bounds, no need to continue heapify, break
+            int l = 2 * i + 1;
+            int r = 2 * i + 2;
+            int ma = i;
+            if (l < n && nums[l] > nums[ma])
+                ma = l;
+            if (r < n && nums[r] > nums[ma])
+                ma = r;
+            // Swap two nodes
+            if (ma == i)
+                break;
+            // Swap two nodes
+            (nums[ma], nums[i]) = (nums[i], nums[ma]);
+            // Loop downwards heapification
+            i = ma;
+        }
+    }
+
+    /* Heap sort */
+    void HeapSort(int[] nums) {
+        // Build heap operation: heapify all nodes except leaves
+        for (int i = nums.Length / 2 - 1; i >= 0; i--) {
+            SiftDown(nums, nums.Length, i);
+        }
+        // Extract the largest element from the heap and repeat for n-1 rounds
+        for (int i = nums.Length - 1; i > 0; i--) {
+            // Delete node
+            (nums[i], nums[0]) = (nums[0], nums[i]);
+            // Start heapifying the root node, from top to bottom
+            SiftDown(nums, i, 0);
+        }
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = [4, 1, 3, 1, 5, 2];
+        HeapSort(nums);
+        Console.WriteLine("After heap sort completes, nums = " + string.Join(" ", nums));
+    }
+}

en/codes/csharp/chapter_sorting/insertion_sort.cs

@@ -0,0 +1,30 @@
+/**
+ * File: insertion_sort.cs
+ * Created Time: 2022-12-23
+ * Author: haptear (haptear@hotmail.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class insertion_sort {
+    /* Insertion sort */
+    void InsertionSort(int[] nums) {
+        // Outer loop: sorted interval is [0, i-1]
+        for (int i = 1; i < nums.Length; i++) {
+            int bas = nums[i], j = i - 1;
+            // Inner loop: insert base into the correct position within the sorted interval [0, i-1]
+            while (j >= 0 && nums[j] > bas) {
+                nums[j + 1] = nums[j]; // Move nums[j] to the right by one position
+                j--;
+            }
+            nums[j + 1] = bas;         // Assign base to the correct position
+        }
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = [4, 1, 3, 1, 5, 2];
+        InsertionSort(nums);
+        Console.WriteLine("After insertion sort completes, nums = " + string.Join(",", nums));
+    }
+}

en/codes/csharp/chapter_sorting/merge_sort.cs

@@ -0,0 +1,56 @@
+/**
+ * File: merge_sort.cs
+ * Created Time: 2022-12-23
+ * Author: haptear (haptear@hotmail.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class merge_sort {
+    /* Merge left subarray and right subarray */
+    void Merge(int[] nums, int left, int mid, int right) {
+        // Left subarray interval is [left, mid], right subarray interval is [mid+1, right]
+        // Create a temporary array tmp to store the merged results
+        int[] tmp = new int[right - left + 1];
+        // Initialize the start indices of the left and right subarrays
+        int i = left, j = mid + 1, k = 0;
+        // While both subarrays still have elements, compare and copy the smaller element into the temporary array
+        while (i <= mid && j <= right) {
+            if (nums[i] <= nums[j])
+                tmp[k++] = nums[i++];
+            else
+                tmp[k++] = nums[j++];
+        }
+        // Copy the remaining elements of the left and right subarrays into the temporary array
+        while (i <= mid) {
+            tmp[k++] = nums[i++];
+        }
+        while (j <= right) {
+            tmp[k++] = nums[j++];
+        }
+        // Copy the elements from the temporary array tmp back to the original array nums at the corresponding interval
+        for (k = 0; k < tmp.Length; ++k) {
+            nums[left + k] = tmp[k];
+        }
+    }
+
+    /* Merge sort */
+    void MergeSort(int[] nums, int left, int right) {
+        // Termination condition
+        if (left >= right) return;       // Terminate recursion when subarray length is 1
+        // Divide and conquer stage
+        int mid = left + (right - left) / 2;    // Calculate midpoint
+        MergeSort(nums, left, mid);      // Recursively process the left subarray
+        MergeSort(nums, mid + 1, right); // Recursively process the right subarray
+        // Merge stage
+        Merge(nums, left, mid, right);
+    }
+
+    [Test]
+    public void Test() {
+        /* Merge sort */
+        int[] nums = [7, 3, 2, 6, 0, 1, 5, 4];
+        MergeSort(nums, 0, nums.Length - 1);
+        Console.WriteLine("After merge sort completes, nums = " + string.Join(",", nums));
+    }
+}

en/codes/csharp/chapter_sorting/quick_sort.cs

@@ -0,0 +1,150 @@
+/**
+ * File: quick_sort.cs
+ * Created Time: 2022-12-23
+ * Author: haptear (haptear@hotmail.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+class quickSort {
+    /* Swap elements */
+    static void Swap(int[] nums, int i, int j) {
+        (nums[j], nums[i]) = (nums[i], nums[j]);
+    }
+
+    /* Sentinel partition */
+    static int Partition(int[] nums, int left, int right) {
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            Swap(nums, i, j); // Swap these two elements
+        }
+        Swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i;             // Return the index of the pivot
+    }
+
+    /* Quick sort */
+    public static void QuickSort(int[] nums, int left, int right) {
+        // Terminate recursion when subarray length is 1
+        if (left >= right)
+            return;
+        // Sentinel partition
+        int pivot = Partition(nums, left, right);
+        // Recursively process the left subarray and right subarray
+        QuickSort(nums, left, pivot - 1);
+        QuickSort(nums, pivot + 1, right);
+    }
+}
+
+/* Quick sort class (median pivot optimization) */
+class QuickSortMedian {
+    /* Swap elements */
+    static void Swap(int[] nums, int i, int j) {
+        (nums[j], nums[i]) = (nums[i], nums[j]);
+    }
+
+    /* Select the median of three candidate elements */
+    static int MedianThree(int[] nums, int left, int mid, int right) {
+        int l = nums[left], m = nums[mid], r = nums[right];
+        if ((l <= m && m <= r) || (r <= m && m <= l))
+            return mid; // m is between l and r
+        if ((m <= l && l <= r) || (r <= l && l <= m))
+            return left; // l is between m and r
+        return right;
+    }
+
+    /* Sentinel partition (median of three) */
+    static int Partition(int[] nums, int left, int right) {
+        // Select the median of three candidate elements
+        int med = MedianThree(nums, left, (left + right) / 2, right);
+        // Swap the median to the array's leftmost position
+        Swap(nums, left, med);
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            Swap(nums, i, j); // Swap these two elements
+        }
+        Swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i;             // Return the index of the pivot
+    }
+
+    /* Quick sort */
+    public static void QuickSort(int[] nums, int left, int right) {
+        // Terminate recursion when subarray length is 1
+        if (left >= right)
+            return;
+        // Sentinel partition
+        int pivot = Partition(nums, left, right);
+        // Recursively process the left subarray and right subarray
+        QuickSort(nums, left, pivot - 1);
+        QuickSort(nums, pivot + 1, right);
+    }
+}
+
+/* Quick sort class (recursion depth optimization) */
+class QuickSortTailCall {
+    /* Swap elements */
+    static void Swap(int[] nums, int i, int j) {
+        (nums[j], nums[i]) = (nums[i], nums[j]);
+    }
+
+    /* Sentinel partition */
+    static int Partition(int[] nums, int left, int right) {
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            Swap(nums, i, j); // Swap these two elements
+        }
+        Swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i; // Return the index of the pivot
+    }
+
+    /* Quick sort (recursion depth optimization) */
+    public static void QuickSort(int[] nums, int left, int right) {
+        // Terminate when subarray length is 1
+        while (left < right) {
+            // Sentinel partition operation
+            int pivot = Partition(nums, left, right);
+            // Perform quick sort on the shorter of the two subarrays
+            if (pivot - left < right - pivot) {
+                QuickSort(nums, left, pivot - 1);  // Recursively sort the left subarray
+                left = pivot + 1;  // Remaining unsorted interval is [pivot + 1, right]
+            } else {
+                QuickSort(nums, pivot + 1, right); // Recursively sort the right subarray
+                right = pivot - 1; // Remaining unsorted interval is [left, pivot - 1]
+            }
+        }
+    }
+}
+
+public class quick_sort {
+    [Test]
+    public void Test() {
+        /* Quick sort */
+        int[] nums = [2, 4, 1, 0, 3, 5];
+        quickSort.QuickSort(nums, 0, nums.Length - 1);
+        Console.WriteLine("After quick sort completes, nums = " + string.Join(",", nums));
+
+        /* Quick sort (recursion depth optimization) */
+        int[] nums1 = [2, 4, 1, 0, 3, 5];
+        QuickSortMedian.QuickSort(nums1, 0, nums1.Length - 1);
+        Console.WriteLine("After quick sort (median pivot optimization) completes, nums1 = " + string.Join(",", nums1));
+
+        /* Quick sort (recursion depth optimization) */
+        int[] nums2 = [2, 4, 1, 0, 3, 5];
+        QuickSortTailCall.QuickSort(nums2, 0, nums2.Length - 1);
+        Console.WriteLine("After quick sort (recursion depth optimization) completes, nums2 = " + string.Join(",", nums2));
+    }
+}

en/codes/csharp/chapter_sorting/radix_sort.cs

@@ -0,0 +1,69 @@
+/**
+ * File: radix_sort.cs
+ * Created Time: 2023-04-13
+ * Author: hpstory (hpstory1024@163.com)
+ */
+
+namespace hello_algo.chapter_sorting;
+
+public class radix_sort {
+    /* Get the k-th digit of element num, where exp = 10^(k-1) */
+    int Digit(int num, int exp) {
+        // Passing exp instead of k can avoid repeated expensive exponentiation here
+        return (num / exp) % 10;
+    }
+
+    /* Counting sort (based on nums k-th digit) */
+    void CountingSortDigit(int[] nums, int exp) {
+        // Decimal digit range is 0~9, therefore need a bucket array of length 10
+        int[] counter = new int[10];
+        int n = nums.Length;
+        // Count the occurrence of digits 0~9
+        for (int i = 0; i < n; i++) {
+            int d = Digit(nums[i], exp); // Get the k-th digit of nums[i], noted as d
+            counter[d]++;                // Count the occurrence of digit d
+        }
+        // Calculate prefix sum, converting "occurrence count" into "array index"
+        for (int i = 1; i < 10; i++) {
+            counter[i] += counter[i - 1];
+        }
+        // Traverse in reverse, based on bucket statistics, place each element into res
+        int[] res = new int[n];
+        for (int i = n - 1; i >= 0; i--) {
+            int d = Digit(nums[i], exp);
+            int j = counter[d] - 1; // Get the index j for d in the array
+            res[j] = nums[i];       // Place the current element at index j
+            counter[d]--;           // Decrease the count of d by 1
+        }
+        // Use result to overwrite the original array nums
+        for (int i = 0; i < n; i++) {
+            nums[i] = res[i];
+        }
+    }
+
+    /* Radix sort */
+    void RadixSort(int[] nums) {
+        // Get the maximum element of the array, used to determine the maximum number of digits
+        int m = int.MinValue;
+        foreach (int num in nums) {
+            if (num > m) m = num;
+        }
+        // Traverse from the lowest to the highest digit
+        for (int exp = 1; exp <= m; exp *= 10) {
+            // Perform counting sort on the k-th digit of array elements
+            // k = 1 -> exp = 1
+            // k = 2 -> exp = 10
+            // i.e., exp = 10^(k-1)
+            CountingSortDigit(nums, exp);
+        }
+    }
+
+    [Test]
+    public void Test() {
+        // Radix sort
+        int[] nums = [ 10546151, 35663510, 42865989, 34862445, 81883077,
+            88906420, 72429244, 30524779, 82060337, 63832996 ];
+        RadixSort(nums);
+        Console.WriteLine("After radix sort completes, nums = " + string.Join(" ", nums));
+    }
+}

en/codes/csharp/chapter_sorting/selection_sort.cs

@@ -0,0 +1,32 @@
+/**
+* File: selection_sort.cs
+* Created Time: 2023-06-01
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_sorting;
+
+public class selection_sort {
+    /* Selection sort */
+    void SelectionSort(int[] nums) {
+        int n = nums.Length;
+        // Outer loop: unsorted interval is [i, n-1]
+        for (int i = 0; i < n - 1; i++) {
+            // Inner loop: find the smallest element within the unsorted interval
+            int k = i;
+            for (int j = i + 1; j < n; j++) {
+                if (nums[j] < nums[k])
+                    k = j; // Record the index of the smallest element
+            }
+            // Swap the smallest element with the first element of the unsorted interval
+            (nums[k], nums[i]) = (nums[i], nums[k]);
+        }
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = [4, 1, 3, 1, 5, 2];
+        SelectionSort(nums);
+        Console.WriteLine("After selection sort completes, nums = " + string.Join(" ", nums));
+    }
+}