leetcode dfs Validate Binary Search Tree

Validate Binary Search Tree

 Total Accepted: 23828 Total Submissions: 91943My Submissions

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，推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下面两个条件
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);
}