leetcode dfs Validate Binary Search Tree

Validate Binary Search Tree

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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.

题意：给定一棵二叉树，判断它是不是合法的二叉查找树
思路：dfs
合法二叉查找树必须满足以下两个条件
1.左子树和右子树都是合法二叉查找树
2.左子树的最右叶子节点 < 根 < 右子树的最左叶子节点
复杂度：时间O(n)，空间O(log n)

bool isValidBST(TreeNode *root) {if(!root) return true;TreeNode *right_most = root->left, *left_most = root->right;while(right_most && right_most->right){right_most = right_most->right;}while(left_most && left_most->left){left_most = left_most->left;}return isValidBST(root->left) && isValidBST(root->right)&& (!right_most || right_most->val < root->val)&& (!left_most || root->val < left_most->val);}