[LeetCode] 958. Check Completeness of a Binary Tree

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<TreeNode> 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;
    }
}