Binary Tree Level Order Traversal @LeetCode

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]
]

	package leetcode.tree;
	import java.util.ArrayList;
	import java.util.LinkedList;
	import java.util.Queue;
	/**
	* Thought: Use Queue to store every layer nodes, the most important is to terminate level. we use queue size.
	*
	* 1. int queueSize = queue.size(), we cannot put queue.size() in for loop as it will changes in loop.
	* 2. Queue is just a interface, we use LinkedList as implement class.
	* 3. Must judge if root == null, or NullPointerProblem in list.add(node.val). return result instead of null -- coding style.
	* Extension: Except this way, any other ways?
	* 1. Queue + dummyNode --> insert dummyNode when one line ends. dummyNode has No meaning like Integer.MAX_VALUE
	* 2. Two Queue --> next time, we stored in another queue, for loop ends until size of current queue size==0.
	*
	* Actually, our ways is the best way (use quese size to control line).
	*
	* @author jeffwan
	* @date Feb 12, 2014
	*/
	public class LevelOrder {
	public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
	ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
	if (root == null) {
	return result;
	}
	Queue<TreeNode> queue = new LinkedList<TreeNode>();
	queue.offer(root);
	while(!queue.isEmpty()) {
	ArrayList<Integer> level = new ArrayList<Integer>();
	int queueSize = queue.size();
	for (int i = 0 ; i < queueSize; i++) {
	TreeNode node = queue.poll();
	level.add(node.val);
	if (node.left != null) {
	queue.offer(node.left);
	}
	if (node.right != null) {
	queue.offer(node.right);
	}
	}
	result.add(level);
	}
	return result;
	}
	// TreeNode
	private class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode (int x) {
	val = x;
	}
	}
	}

view raw LevelOrder.java hosted with ❤ by GitHub