leetcode-98-Validate-Binary-Search-Tree

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描述

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.

分析

判断一个二叉树是否为二分查找树。
需要注意的是，如果是左子树，是左子树的所有节点都比根节点小，而非只是左孩子比其小。

解决方案1(C++)

遍历时记录当前节点所允许的最大值、最小值，时间复杂度O(n)，空间复杂度O(logn)

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/**
 * 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:
    bool isValidBST(TreeNode* root) {
        return isValidBST(root, LLONG_MIN, LLONG_MAX);
    }
    
    bool isValidBST(TreeNode* root, long lower, long upper) {
        if(root == NULL) {
            return true;
        }
        
        return root->val>lower && root->val<upper && 
                isValidBST(root->left, lower, root->val) &&
                isValidBST(root->right, root->val, upper);
    }
};

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