leetcode dfs Validate Binary Search Tree

时间:2022-03-08 11:17:48

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);
}