102. Binary Tree Level Order Traversal

Easy (LinkedIn, Facebook, Amazon, Microsoft, Apple, Bloomberg)

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example: Given binary tree [3,9,20,null,null,15,7], return its level order traversal as: [ [3], [9,20], [15,7] ]

/**

• Definition for a binary tree node.
• public class TreeNode {
• int val;
• TreeNode left;
• TreeNode right;
• TreeNode(int x) { val = x; }
• } */

public class Solution {

// test case:
// root is empty

public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> ans = new ArrayList<>();

    if(root == null) {
        return ans;
    }

    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    int size = queue.size();
    List<Integer> list = new ArrayList<>();

    while(!queue.isEmpty()) {
        TreeNode node = queue.poll();
        list.add(node.val);

        if(node.left != null) {
            queue.offer(node.left);
        } 

        if(node.right != null) {
            queue.offer(node.right);
        }

        if(--size == 0) {
            size = queue.size();
            ans.add(new ArrayList<Integer>(list));
            list.clear();
        }
    }

    return ans;
 }

}