lc98-Validate Binary Search Tree

發表於   |   分類於   |   0 comments   |  

要注意的是rely on INT_MIN的问题。

in-order:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
/**
 * 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 {
    TreeNode *prev = NULL;
public:
    bool isValidBST(TreeNode* root) {
        if (!root) return true;
        if (root->left) if (!isValidBST(root->left)) return false;;
        if (prev && root->val <= prev->val)
            return false;
        prev = root;
        if (root->right) if (!isValidBST(root->right)) return false;
        return true;
    }
};

pre-order:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
/**
 * 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 helper(root, nullptr, nullptr);
    }
    
    bool helper(TreeNode* root, TreeNode* minNode, TreeNode* maxNode) {
        if (!root) return true;
        if ((minNode && root->val <= minNode->val) || (maxNode && root->val >= maxNode->val))
            return false;
        return helper(root->left, minNode, root) && helper(root->right, root, maxNode);
    }
};