Problem

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 2^h nodes inclusive at the last level h.

Solution

class Solution {    public boolean isCompleteTree(TreeNode root) {        if (root == null) return true;        Queue
    
      queue = new LinkedList<>();        queue.offer(root);        boolean lastLevel = false;        while (!queue.isEmpty()) {            TreeNode cur = queue.poll();            if (cur == null) lastLevel = true;            else {                if (lastLevel) return false;                queue.offer(cur.left);                queue.offer(cur.right);            }        }        return true;    }}