【题解】【BST】【Leetcode】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.

思路：

这题其实考查的是BST的定义，下面code的是CC150v5上的第一种答案简化版，同样借鉴中序遍历的思想，但是只保存当前点上家的值，判断当前点是否大于上家。

代码：

 bool isValidBST(TreeNode *root) {
     int init = INT_MIN;
     return isValidNode(root, init);
 }
 bool isValidNode(TreeNode *root, int& lastVal){
     if(root == NULL)
         return true;

     if(!isValidNode(root->left, lastVal))
         return false;

     if(root->val <= lastVal) //不是<而是<=，failed {1,1}=>false
         return false;
     lastVal = root->val;

     if(!isValidNode(root->right, lastVal))
         return false;

     return true;//忘写最后一句了
 }