958. Check Completeness of a Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
// Refer : https://leetcode.com/problems/check-completeness-of-a-binary-tree/discuss/205682/JavaC++Python-BFS-Level-Order-Traversal
/*
Time Complexity: O(N)
Space Complexity: O(N)

N = Number of nodes;
*/
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        if (root == null) {
            return true;
        }

        Queue<TreeNode> queue = new LinkedList();
        queue.offer(root);

        while (queue.peek() != null) {
            TreeNode node = queue.poll();
            queue.offer(node.left);
            queue.offer(node.right);
        }

        while (!queue.isEmpty() && queue.peek() == null) {
            queue.poll();
        }

        return queue.isEmpty();
    }
}


/*
Time Complexity: O(N)
Space Complexity: O(N)

N = Number of nodes;
*/
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        if (root == null) {
            return true;
        }

        Queue<TreeNode> queue = new LinkedList();
        queue.offer(root);

        while (true) {
            TreeNode node = queue.poll();
            if (node.left == null) {
                if (node.right != null) {
                    return false;
                }
                break;
            }
            queue.offer(node.left);
            if (node.right == null) {
                break;
            }
            queue.offer(node.right);
        }

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (node.left != null || node.right != null) {
                return false;
            }
        }

        return true;
    }
}