Leetcode_Validate Binary Search Tree

標籤：leetcode   資料結構   演算法   

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.

  class Solution {public:    void  isValid(TreeNode *root ,int &pre,bool &s) {        if(root==NULL)        return ;        else{                   isValid(root->left,pre ,s);                  if(root->val <= pre ){              s = false;              return;          }          pre = root->val;          isValid(root->right,pre,s);                                      }            }    bool isValidBST(TreeNode *root) {        bool s=true;        int pre=-2000000;         isValid(root,pre,s);        return s;    }};