[面试真题] 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.

解法：递归地判断当前节点是否大于左孩子，并且是否大于其左孩子的每一个右子孙；同理，判断当前节点是否小于右孩子，并且是否小于其右孩子的每一个左子孙

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(NULL == root){
            return true;
        }
        if(root->left){
            TreeNode *p = root->left;
            if(root->val <= p->val){
                    return false;        
                }
            while(p->right){
                if(root->val <= p->right->val){
                    return false;        
                }
                p = p->right;
            }
        }
        if(root->right){
            TreeNode *p = root->right;
            if(root->val >= p->val){
                    return false;        
                }
            while(p->left){
                if(root->val >= p->left->val){
                    return false;        
                }
                p = p->left;
            }
        }
        return isValidBST(root->left) && isValidBST(root->right);
        
    }
};

Run Status: Accepted!
Program Runtime: 68 milli secs

Progress: 54/54 test cases passed.