【leetcode】 Validate Binary Search Tree
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.
confused what "{1,#,2,3}"
means? > read more on how binary tree is serialized on OJ.
OJ's Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
Here's an example:
1
/ \
2 3
/
4
\
5
The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}"
.
解体思路:中序遍历BST得到的递增的数组,可以使用栈中序遍历得到数组,比较数组元素来判断是否是BST。
class Solution {
public:
bool isValidBST(TreeNode *root) {
vector<TreeNode*> stack;
TreeNode* node = root;
vector<int> v;
while (stack.size()> || node!=NULL) {//inorder
if (node!=NULL){
stack.push_back(node);
node = node->left;
}else{
node = stack.back();
stack.pop_back();
v.push_back(node->val);
node = node->right;
}
} for(int i=; v.size()> && i<v.size()-; i++)
if (v[i] >= v[i+])
return false; return true;
}
};
疑惑:以下代码采用递归,但是有测试用例通不过,没找到原因。
/**
* Definition for binary tree
* 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, INT_MIN, INT_MAX);
} bool isValidBST(TreeNode *root, int lower, int upper){
if(root == nullptr)
return true;
return root->val >= lower && root->val <= upper && isValidBST(root->left, lower, root->val)
&& isValidBST(root->right, root->val, upper);
}
};
67 / 74 test cases passed.
|
Status:
Wrong Answer |
Submitted: 3 hours, 26 minutes ago
|
Input: | {2147483647} |
Output: | false |
Expected: | true |
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