feat: add the solution of Two Sum IV - Input is a BST(653) with Java.

Author: Mukyu

README.md

@@ -64,7 +64,7 @@
 | [561][561-question] | [Array Partition I][561-tips]                                | [✅][561-java] | [✅][561-js] |                  |
 | [643][643-question] | [Maximum Average Subarray I][643-tips]                       | [✅][643-java] | [✅][643-js] |                  |
 | [645][645-question] | [Set Mismatch][645-tips]                                     | [✅][645-java] | [✅][645-js] |                  |
-| [653][653-question] | [Two Sum IV - Input is a BST][653-tips]                      |                | [✅][653-js] |                  |
+| [653][653-question] | [Two Sum IV - Input is a BST][653-tips]                      | [✅][653-java] | [✅][653-js] |                  |
 | [657][657-question] | [Judge Route Circle][657-tips]                               |                | [✅][657-js] |                  |
 | [661][661-question] | [Image Smoother][661-tips]                                   |                | [✅][661-js] |                  |
 | [665][665-question] | [Non-decreasing Array][665-tips]                             |                | [✅][665-js] |                  |
@@ -455,6 +455,7 @@
 [561-java]: ./src/_561/Solution.java
 [643-java]: ./src/_643/Solution.java
 [645-java]: ./src/_645/Solution.java
+[653-java]: ./src/_653/Solution.java
 [728-java]: ./src/_728/Solution.java
 [771-java]: ./src/_771/Solution.java
 [804-java]: ./src/_804/Solution.java

src/_653/Solution.java

@@ -0,0 +1,59 @@
+package _653;
+
+import java.util.Stack;
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public boolean findTarget(TreeNode root, int k) {
+        Stack<TreeNode> stack = new Stack<>();
+        int num;
+        TreeNode next;
+        stack.push(root);
+        while (!stack.empty()) {
+            TreeNode node = stack.pop();
+
+            next = root;
+            num = k - node.val;
+            while (next != null) {
+                if (next.val == num) {
+                    if (next == node) {
+                        break;
+                    } else {
+                        return true;
+                    }
+                } else if (num > next.val) {
+                    next = next.right;
+                } else {
+                    next = next.left;
+                }
+            }
+
+            if (node.left != null) {
+                stack.push(node.left);
+            }
+            if (node.right != null) {
+                stack.push(node.right);
+            }
+        }
+
+        return false;
+    }
+
+    class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+
+        TreeNode(int x) {
+            val = x;
+        }
+    }
+}

tips/653/README.md

@@ -1,46 +1,91 @@
-[xxxx][title]
+[Two Sum IV - Input is a BST][title]
 
 ## Description
-// 抄题目
+Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.
 
+**Example 1:**
 
-**Example:**
-
-```
-// 抄Example
 ```
+Input: 
+    5
+   / \
+  3   6
+ / \   \
+2   4   7
 
-**Note:**
-// Note
-
-**Tags:** // tags
+Target = 9
 
+Output: True
+```
 
-## 思路 1
-// 贴一些关键代码,说一些解题思路
-// (同一种语言可以写多种思路，与某种语言思路相同的另一种语言的思路无须赘述，但可以把代码贴在后面) 
-```java
+**Example 2:**
 
 ```
-```javascript
+Input: 
+    5
+   / \
+  3   6
+ / \   \
+2   4   7
+
+Target = 28
 
+Output: False
 ```
 
-## 思路 2
-// 贴一些关键代码,说一些解题思路
-```java
+**Tags:** [Tree](https://leetcode.com/tag/tree/)
 
-```
+## 思路
+
+题目要求在搜索二叉树中找到两个数，使其和等于所给值。此题主要考验搜索二叉树的用法。先遍历二叉树，在遍历过程中利用搜索二叉树的特性，查找是否存在另一个节点的值使两个节点的和等于所给的值。例如在遍历到节点a时，查找二叉树中是否存在节点b使a+b=k成立。
 
-## 思路 3
-// 贴一些关键代码,说一些解题思路
-```kotlin
+**Java**
 
+```java
+class Solution {
+    public boolean findTarget(TreeNode root, int k) {
+        Stack<TreeNode> stack = new Stack<>();
+        int num;
+        TreeNode next;
+        
+        // 遍历二叉树
+        stack.push(root);
+        while (!stack.empty()) {
+            TreeNode node = stack.pop();
+
+            // 查找二叉树
+            next = root;
+            num = k - node.val;
+            while (next != null) {
+                if (next.val == num) {
+                    if (next == node) {
+                        break;
+                    } else {
+                        return true;
+                    }
+                } else if (num > next.val) {
+                    next = next.right;
+                } else {
+                    next = next.left;
+                }
+            }
+
+            if (node.left != null) {
+                stack.push(node.left);
+            }
+            if (node.right != null) {
+                stack.push(node.right);
+            }
+        }
+
+        return false;
+    }
+}
 ```
 
 ## 结语
-   
+
 如果你同我们一样热爱数据结构、算法、LeetCode，可以关注我们 GitHub 上的 LeetCode 题解：[LeetCode-Solution][ls]
 
-[title]: https://leetcode.com/problems/xxxx
+[title]: https://leetcode.com/problems/two-sum-iv-input-is-a-bst/description/
 [ls]: https://github.com/SDE603/LeetCode-Solution