Add Solution for 098 Validate Binary Search Tree

cpp/098_Validate_Binary_Search_Tree.cpp

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+// 98. Validate Binary Search Tree
+/**
+ * Given a binary tree, determine if it is a valid binary search tree (BST).
+ * 
+ * Assume a BST is defined as follows:
+ * 
+ * The left subtree of a node contains only nodes with keys less than the node's key.
+ * The right subtree of a node contains only nodes with keys greater than the node's key.
+ * Both the left and right subtrees must also be binary search trees.
+ * 
+ * Example 1:
+ *     2
+ *    / \
+ *   1   3
+ * Binary tree [2,1,3], return true.
+ * 
+ * Example 2:
+ *     1
+ *    / \
+ *   2   3
+ * Binary tree [1,2,3], return false.
+ * 
+ * Tags: Tree, Depth-first Search
+ * 
+ * Similar Problems: (M) Binary Tree Inorder Traversal, (E) Find Mode in Binary Search Tree
+ * 
+ * Author: Kuang Qin
+ */
+
+#include <iostream>
+
+using namespace std;
+
+/**
+ * Definition for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+    TreeNode(int x, TreeNode *l, TreeNode *r) : val(x), left(l), right(r) {}
+};
+
+class Solution {
+    bool validate(TreeNode* root, TreeNode *minNode, TreeNode *maxNode) {
+        if (root == NULL) {
+            return true;
+        }
+
+        // root smaller than minNode
+        if (minNode != NULL && root->val <= minNode->val) {
+            return false;
+        }
+
+        // root larger than maxNode
+        if (maxNode != NULL && root->val >= maxNode->val) {
+            return false;
+        }
+
+        return validate(root->left, minNode, root) && validate(root->right, root, maxNode);
+    }
+public:
+    bool isValidBST(TreeNode* root) {
+        return validate(root, NULL, NULL);
+    }
+};
+
+int main() {
+    TreeNode node1(1), node4(4), node2(2, &node1, NULL), node5(5, &node4, NULL), node3(3, &node2, &node5);
+    Solution sol;
+    bool ans = sol.isValidBST(&node3);
+    cout << ans << endl;
+    cin.get();
+    return 0;
+}