feat: #98 add C++ implementation for in-order (#102)

problems/98.validate-binary-search-tree.md

@@ -54,7 +54,7 @@ Explanation: The root node's value is 5 but its right child's value is 4.
 
 ## 代码
 ### 中序遍历
-* 语言支持：JS
+* 语言支持：JS，C++
 
 JavaScript Code：
 ```js
@@ -104,6 +104,48 @@ var isValidBST = function(root) {
     return true;
 };
 
+```
+
+C++ Code：
+```c++
+// 递归
+class Solution {
+public:
+    bool isValidBST(TreeNode* root) {
+        TreeNode* prev = nullptr;
+        return validateBstInorder(root, prev);
+    }
+
+private:
+    bool validateBstInorder(TreeNode* root, TreeNode*& prev) {
+        if (root == nullptr) return true;
+        if (!validateBstInorder(root->left, prev)) return false;
+        if (prev != nullptr && prev->val >= root->val) return false;
+        prev = root;
+        return validateBstInorder(root->right, prev);
+    }
+};
+
+// 迭代
+class Solution {
+public:
+    bool isValidBST(TreeNode* root) {
+        auto s = vector<TreeNode*>();
+        TreeNode* prev = nullptr;
+        while (root != nullptr || !s.empty()) {
+            while (root != nullptr) {
+                s.push_back(root);
+                root = root->left;
+            }
+            root = s.back();
+            s.pop_back();
+            if (prev != nullptr && prev->val >= root->val) return false;
+            prev = root;
+            root = root->right;
+        }
+        return true;
+    }
+};
 ```
 ### 定义法
 * 语言支持：C++