Validate Binary Search Tree

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.

看似简单，纠结了好久

<span style="font-size:18px;">bool isValidBST(TreeNode *root)    {       return isValidBST(root,INT_MIN,INT_MAX);    }        bool isValidBST(TreeNode* root,int lower,int upper)    {        if(root ==nullptr) return true;                return root->val>lower&&root->val<upper&&isValidBST(root->left,lower,root->val)&&isValidBST(root->right,root->val,upper);    }</span>