[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.

 

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}".

 

遞歸判斷，遞歸時傳入兩個參數，一個是左界，一個是右界，節點的值必須在兩個界的中間，同時在判斷做子樹和右子樹時更新左右界。

/**
 * 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 check(TreeNode *node, int leftVal, int rightVal)
    {
        if (node == NULL)
            return true;
            
        return leftVal < node->val && node->val < rightVal && check(node->left, leftVal, node->val) &&
            check(node->right, node->val, rightVal);
    }
    
    bool isValidBST(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return check(root, INT_MIN, INT_MAX);        
    }
};