【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
}