leetcode-98-Validate-Binary-Search-Tree

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);
}
};

相关问题


(M) Binary Tree Inorder Traversal

题目来源