LeetCode题解: Validate Binary Search Tree

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:

仍然是树的深度优先搜索。这里要注意的是,不能仅仅和父结点的值比较,还必须和上面所有的结点进行比较。

题解:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<pair<int, char>> history;

    bool isValidBST(TreeNode *node) {
        if (node == nullptr)
            return true;
            
        // check if I am ok with all the parents
        for(auto& h : history)
            if (h.second == 'R')
            {
                // i am at the right subtree
                if (node->val <= h.first)
                    return false;
            }
            else
            {
                // i am at the left subtree
                if (node->val >= h.first)
                    return false;
            }
    
        bool leftok = true;
        history.push_back(make_pair(node->val, 'L'));
        leftok = isValidBST(node->left);

        bool rightok = true;
        history.back().second = 'R';
        rightok = isValidBST(node->right);

        history.pop_back();

        return leftok && rightok;
    }
};


你可能感兴趣的:(LeetCode)