hello-algo/en/codes/java/chapter_searching/binary_search.java

/**
 * File: binary_search.java
 * Created Time: 2022-11-25
 * Author: krahets (krahets@163.com)
 */

package chapter_searching;

public class binary_search {
    /* Binary search (closed interval) */
    static int binarySearch(int[] nums, int target) {
        // Initialize the closed interval [0, n-1], with i and j pointing to the first and last elements
        int i = 0, j = nums.length - 1;
        // Loop until the search interval is empty (when i > j)
        while (i <= j) {
            int m = i + (j - i) / 2; // Calculate the midpoint index m
            if (nums[m] < target) // In this case, target must be in [m+1, j] if it exists
                i = m + 1;
            else if (nums[m] > target) // In this case, target must be in [i, m-1] if it exists
                j = m - 1;
            else // The target element was found; return its index
                return m;
        }
        // The target element was not found; return -1
        return -1;
    }

    /* Binary search (left-closed, right-open interval) */
    static int binarySearchLCRO(int[] nums, int target) {
        // Initialize the left-closed, right-open interval [0, n), with i at the first element and j one past the last
        int i = 0, j = nums.length;
        // Loop until the search interval is empty (when i = j)
        while (i < j) {
            int m = i + (j - i) / 2; // Calculate the midpoint index m
            if (nums[m] < target) // In this case, target must be in [m+1, j) if it exists
                i = m + 1;
            else if (nums[m] > target) // In this case, target must be in [i, m) if it exists
                j = m;
            else // The target element was found; return its index
                return m;
        }
        // The target element was not found; return -1
        return -1;
    }

    public static void main(String[] args) {
        int target = 6;
        int[] nums = { 1, 3, 6, 8, 12, 15, 23, 26, 31, 35 };

        /* Binary search (closed interval) */
        int index = binarySearch(nums, target);
        System.out.println("Index of target element 6 = " + index);

        /* Binary search (left-closed, right-open interval) */
        index = binarySearchLCRO(nums, target);
        System.out.println("Index of target element 6 = " + index);
    }
}