feat: Revised the book (#978)

* Sync recent changes to the revised Word.

* Revised the preface chapter

* Revised the introduction chapter

* Revised the computation complexity chapter

* Revised the chapter data structure

* Revised the chapter array and linked list

* Revised the chapter stack and queue

* Revised the chapter hashing

* Revised the chapter tree

* Revised the chapter heap

* Revised the chapter graph

* Revised the chapter searching

* Reivised the sorting chapter

* Revised the divide and conquer chapter

* Revised the chapter backtacking

* Revised the DP chapter

* Revised the greedy chapter

* Revised the appendix chapter

* Revised the preface chapter doubly

* Revised the figures

codes/java/chapter_searching/two_sum.java

@@ -12,7 +12,7 @@ public class two_sum {
     /* 方法一：暴力枚举 */
     static int[] twoSumBruteForce(int[] nums, int target) {
         int size = nums.length;
-        // 两层循环，时间复杂度 O(n^2)
+        // 两层循环，时间复杂度为 O(n^2)
         for (int i = 0; i < size - 1; i++) {
             for (int j = i + 1; j < size; j++) {
                 if (nums[i] + nums[j] == target)
@@ -25,9 +25,9 @@ public class two_sum {
     /* 方法二：辅助哈希表 */
     static int[] twoSumHashTable(int[] nums, int target) {
         int size = nums.length;
-        // 辅助哈希表，空间复杂度 O(n)
+        // 辅助哈希表，空间复杂度为 O(n)
         Map<Integer, Integer> dic = new HashMap<>();
-        // 单层循环，时间复杂度 O(n)
+        // 单层循环，时间复杂度为 O(n)
         for (int i = 0; i < size; i++) {
             if (dic.containsKey(target - nums[i])) {
                 return new int[] { dic.get(target - nums[i]), i };