958. Check Completeness of a Binary Tree

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

📋 Đề Bài

Given the root of a binary tree, determine if it is a complete binary tree.

In a complete binary tree, every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

 

Example 1:

Input: root = [1,2,3,4,5,6]
Output: true
Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

Input: root = [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 100].
  • 1 <= Node.val <= 1000

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)
⏱️ Thời gian O(n)
💾 Không gian O(1)

💻 Lời Giải

C++ 0958-check-completeness-of-a-binary-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 {
public:
    bool isCompleteTree(TreeNode* root) {
        if (!root) {
            return true;
        }
        queue<TreeNode*> mq;
        mq.push(root);
        bool flag = true;
        while (!mq.empty()) {
            TreeNode *curr = mq.front();
            mq.pop();
            if (curr == nullptr) {
                flag = false;
            }
            else {
                if (!flag) {
                    return false;
                }
                mq.push(curr->left);
                mq.push(curr->right);
            }
        }
        return true;
    }
};