LeetCode 98. Validate Binary Search Tree

LeetCode 98. Validate Binary Search Tree

Solution1:我的答案1
法1：利用中序遍历

/**
 * 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) {
        vector<int> res;
        Inordertraversal(root, res);
        if (res.size() <= 1) return true;
        for (int i = 1; i < res.size(); i++) {
            if (res[i-1] >= res[i])
                return false;
        }
        return true;
    }

    void Inordertraversal(TreeNode* root, vector<int>& res) {
        if (!root) return;
        Inordertraversal(root->left, res);
        res.push_back(root->val);
        Inordertraversal(root->right, res);
    }
};

Solution2:
法2：直接递归判断
参考网址：http://www.cnblogs.com/grandyang/p/4298435.html
这个思路好啊~

// Recursion without inorder traversal
class Solution {
public:
    bool isValidBST(TreeNode *root) {
        return isValidBST(root, LONG_MIN, LONG_MAX);
    }
    bool isValidBST(TreeNode *root, long mn, long mx) {
        //在root->val在这个范围内return true，否则return false
        if (!root) return true;
        if (root->val <= mn || root->val >= mx) return false;
        return isValidBST(root->left, mn, root->val) && isValidBST(root->right, root->val, mx);
    }
};