Binary Tree Level Order Traversal

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},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

Solution:
It is actually a bfs(breath first search) question. For bfs, we need a queue to maintain nodes. How can we know how many nodes in the next level? use just a variable to record it and update it level by level. hash map <node - level> is also a choice.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        ArrayList<TreeNode> queue = new ArrayList<TreeNode>();
        if(root == null) return results;
        queue.add(root);
        int currentLevelNum = 1; // tree node number in the current level
        int nextLevelNum = 0;    // tree node number in the next level
        while(queue.size() > 0){
            ArrayList<Integer> oneLevel = new ArrayList<Integer>();
            while(currentLevelNum > 0){
                TreeNode currentNode = queue.get(0);
                if(currentNode.left != null){
                    queue.add(currentNode.left);
                    nextLevelNum++;
                }
                if(currentNode.right != null){
                    queue.add(currentNode.right);
                    nextLevelNum++;
                }
                oneLevel.add(currentNode.val);//add the cur level nodes in result
                currentLevelNum--;
                queue.remove(0);   // dequeue
            }
            results.add(oneLevel);
            currentLevelNum = nextLevelNum;  // update
            nextLevelNum = 0;
        }
       
        return results;
    }
}