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.
/** * Definition for binary tree * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: bool isValidBST(TreeNode *root) { vector<int> InOrderVal; InOrderTraverse(root,InOrderVal); int len = InOrderVal.size(); for(int i=1;i<len;i++){ if(InOrderVal[i]<=InOrderVal[i-1]) return false; } return true; } private: void InOrderTraverse(TreeNode *p,vector<int> &res){ if(p!=NULL){ InOrderTraverse(p->left,res); res.push_back(p->val); InOrderTraverse(p->right,res); } } };