c_cpp 验证二进制搜索Treehttp：//oj.leetcode.com/problems/validate-binary-search-tree/

Posted 2021-05-14

/**
 * 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) {
        if(root == NULL){
            return true;
        }
        else if(!postVisitBST(root).empty()){
            return true;
        }
        else{
            return false;
        }
        
    }
    //return empty vector if it is not BST, and root should not be NULL
    vector<int> postVisitBST(TreeNode *root){
        vector<int> result;
        if(root == NULL){
            //This should not happen!!
            return result;
        }
        else if(root->left == NULL && root->right == NULL){
            result.push_back(root->val);
            result.push_back(root->val);
            return result;
        }
        else if(root->left == NULL){
            vector<int> t_right = postVisitBST(root->right);
            if(t_right.empty()){
                return result;
            }
            if(t_right[0] > root->val){
                result.push_back(root->val);
                result.push_back(t_right[1]);
                return result;
            }
            else{
                return result;
            }
        }
        else if(root->right == NULL){
            vector<int> t_left = postVisitBST(root->left);
            if(t_left.empty()){
                return result;
            }
            if(t_left[1] < root->val){
                result.push_back(t_left[0]);
                result.push_back(root->val);
                return result;
            }
            else{
                return result;
            }
        }
        else{
            vector<int> t_left = postVisitBST(root->left);
            vector<int> t_right = postVisitBST(root->right);
            if(t_left.empty() || t_right.empty()){
                return result;
            }
            if(t_left[1] < root->val && t_right[0] > root->val){
                result.push_back(t_left[0]);
                result.push_back(t_right[1]);
                return result;
            }
            else{
                return result;
            }
        }
        
    }
};