LeetCode 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.

answer:

class Solution {public:    bool isValidBST(TreeNode* root) {        bool flag = true;        TreeNode * pre = NULL;        myVBST(root,pre,flag);        return flag;    }        void myVBST(TreeNode * root,TreeNode * & pre,bool & flag){        if(root == NULL || !flag) return;        myVBST(root->left,pre,flag);        if(pre != NULL && pre->val >= root->val ){            flag = false;            return;        }        pre = root;        myVBST(root->right,pre,flag);    }};