[LeetCode] 958. Check Completeness of a Binary Tree

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

二叉树的完全性检验。

给定一个二叉树的 root ，确定它是否是一个 完全二叉树 。

在一个 完全二叉树 中，除了最后一个关卡外，所有关卡都是完全被填满的，并且最后一个关卡中的所有节点都是尽可能靠左的。它可以包含 1 到 2h 节点之间的最后一级 h 。

来源：力扣（LeetCode）
链接：https://leetcode.cn/problems/check-completeness-of-a-binary-tree
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思路是 BFS 层序遍历，我参考了这个帖子。注意题目定义，对于一个完全二叉树，我们只能允许他在最后一行上有缺失 node，同时在每一行上，node 应该尽量往左。这里我们想到 BFS 不难，但是需要抓到 BFS 的重点：BFS 遍历是从上到下，每一层从左往右遍历的。所以当我们遇到一个空节点的时候，如果在遍历结束前再遇到一个非空节点，则说明这棵树不是完全二叉树。

时间O(n)

空间O(n)

Java实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode prev = root;
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            if (prev == null && cur != null) {
                return false;
            }
            if (cur != null) {
                queue.add(cur.left);
                queue.add(cur.right);
            }
            prev = cur;
        }
        return true;
    }
}

 

LeetCode 题目总结