Binary Tree Level Order Traversal II @LeetCode

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

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

	package leetcode.tree;
	import java.util.ArrayList;
	import java.util.LinkedList;
	import java.util.Queue;
	/**
	* Thought: Almost same to levelOrder BFS I.
	*
	* Only difference is result.add(0, level); every time, we add the next level to top of result.
	* Use LinkedList add(inddex, element);
	*
	* @author jeffwan
	* @date Feb 12, 2014
	*/
	public class LevelOrderBottom {
	public ArrayList<ArrayList<Integer>> levelOrderBottom(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(0, level);
	}
	return result;
	}
	// TreeNode
	private class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode (int x) {
	val = x;
	}
	}
	}

view raw LevelOrderBottom.java hosted with ❤ by GitHub