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,#,#,15,7},

Binary Tree Level Order Traversal -Java-

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) return res;
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        
        while(!queue.isEmpty()) {
            ArrayList level = new ArrayList();
            int size = queue.size();
            
            //Since the items that exist in the queue right now 
            // all are of the  same level, we can do the following
            for (int i =0; i < size; i++) {
                TreeNode curr = queue.poll();
                level.add(curr.val);
                if (curr.left != null) queue.offer(curr.left);
                if (curr.right != null) queue.offer(curr.right);
            }
            res.add(level);
        }
        return res;
    } 
}