98. Validate Binary Search Tree

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)
⏱️ Thời gian O(n²)
💾 Không gian O(n)

💻 Lời Giải

C++ 0098-validate-binary-search-tree.cpp 
/**
 * 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 {
private:
    vector<int> sav;
public:
    void LNR(TreeNode *root) {
        if (!root) {
            return;
        }
        LNR(root->left);
        sav.push_back(root->val);
        LNR(root->right);
    }
    bool isValidBST(TreeNode* root) {
        LNR(root);
        for (int i = 1; i < (int)sav.size(); ++i) {
            if (sav[i - 1] >= sav[i]) {
                return false;
            }
        }
        return true;
    }
};