leetcode98 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.

思路：

给子树确定上下界，判断子树根节点值是否在上下界范围内，然后更新上下界。递归实现。

/**
 * 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) {
        if(!root)
            return true;
        if(judge(root->left,2,0,root->val)&&judge(root->right,1,root->val,0))
            return true;
        return false;
    }
    
    bool judge(TreeNode*root,int flag,int a,int b)//flag=0,小于大于；flag=1,大于；flag=2,小于
    {
        if(!root)
            return true;
        if(flag==0)
        {
            if(!(root->val<b&&root->val>a))
                return false;
            if(judge(root->left,0,a,root->val)&&judge(root->right,0,root->val,b))
                return true;
            return false;
        }
        if(flag==1)
        {
            if(!(root->val>a))
                return false;
            if(judge(root->left,0,a,root->val)&&judge(root->right,1,root->val,0))
                return true;
            return false;
        }
        if(flag==2)
        {
            if(!(root->val<b))
                return false;
            if(judge(root->left,2,0,root->val)&&judge(root->right,0,root->val,b))
                return true;
            return false;
        }
    }
};