LeetCode98 Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST). （Medium）

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.

Example 1:

    2
   / 
  1   3

Binary tree [2,1,3], return true.

Example 2:

    1
   / 
  2   3

Binary tree [1,2,3], return false.

分析：

是否是二叉查找树和中序遍历是否以此递增是等价的，所以可以先做中序遍历，然后将结果数组做判断，注意判断空树。

代码：

/**
 * 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 {
private:
    vector<int> result;
    void helper(TreeNode* root) {
        if (root == nullptr) {
            return;
        }
        helper(root -> left);
        result.push_back(root -> val);
        helper(root -> right);
    }
public:
    bool isValidBST(TreeNode* root) {
         helper(root);
         if (result.size() == 0) {
             return true;;
         }
         for (int i = 0; i < result.size() - 1; ++i) {
             if (result[i] >= result[i + 1])  {
                 return false;
             }
         }
         return true;
    }
};