Likou Selected Top Interview Questions--------Verify Binary Search Tree

Question link
Idea: This
question mainly uses in-order traversal. Of course, as described in the previous question, there are several traversal methods. Only the recursive version is written here. In fact, you can also write a non-recursive version. Use a variable prev to save the last accessed value, and then judge it by comparing the conditions.

Code:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    
    
public:
    long long prev = LONG_MIN;
    bool dfs(TreeNode* root){
    
    
        if(root==NULL) return true;
        if(dfs(root->left)==false) return false;  //判断左子树
        if(root->val <= prev) return false;
        prev = root->val;
        if(dfs(root->right)==false) return false;  //判断右子树
        else return true;
    }
    bool isValidBST(TreeNode* root) {
    
    
        return dfs(root);
    }


};