958. Check Completeness of a Binary Tree

本文介绍了一种判断给定二叉树是否为完全二叉树的算法。完全二叉树定义为除最后一层外每层都完全填满，且最后一层所有节点尽可能靠左。算法使用队列进行层次遍历，引入标志位来提高效率，时间复杂度为O(n)，空间复杂度为O(n)。

Given a binary tree, determine if it is a complete binary tree.

Definition of a complete binary tree from Wikipedia:
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: [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: [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.

Note:

The tree will have between 1 and 100 nodes.

用一个boolean flag表示气泡，一旦遇到气泡就可以不用再看其他节点

time: O(n), space: O(n)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        if(root == null) {
            return true;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        boolean flag = false;
        while(!q.isEmpty()) {
            TreeNode cur = q.poll();
            if(cur.left == null) {
                flag = true;
            } else if(flag) {
                return false;
            } else {
                q.offer(cur.left);
            }
            
            if(cur.right == null) {
                flag = true;
            } else if(flag) {
                return false;
            } else {
                q.offer(cur.right);
            }
        }
        return true;
    }
}