day17-xxx.md

/**

• Definition for a binary tree node.

• struct TreeNode {

• int val;
  

• TreeNode *left;
  

• TreeNode *right;
  

• TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  

• }; / class Solution { public: int min(TreeNode root) { while(root->left) { root = root->left; } return root->val; } int max(TreeNode* root) { while(root->right){ root = root -> right; } return root->val; } bool isValidBST(TreeNode* root) {

   bool left = true;
   bool right = true;
   if(root) {
       int val = root->val;
       if(root -> left){
           left = isValidBST(root->left) && (val>max(root->left));
       }
       if(root -> right) {
     
          right = isValidBST(root->right)&&(val<min(root->right));
   
        }
   if (!root->left && !root->right) {
           return true;
   }
   return left&&right;
  

  } return true; }

};