_107.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**107. Binary Tree Level Order Traversal II

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

public class _107 {
	public static class Solution1 {
		public List<List<Integer>> levelOrder(TreeNode root) {
			List<List<Integer>> result = new ArrayList();
			if (root == null) {
				return result;
			}

			Queue<TreeNode> q = new LinkedList();
			q.offer(root);
			while (!q.isEmpty()) {
				List<Integer> thisLevel = new ArrayList<Integer>();
				int qSize = q.size();
				for (int i = 0; i < qSize; i++) {
					TreeNode curr = q.poll();
					thisLevel.add(curr.val);
					if (curr.left != null) {
						q.offer(curr.left);
					}
					if (curr.right != null) {
						q.offer(curr.right);
					}
				}
				result.add(thisLevel);
			}
			Collections.reverse(result);
			return result;
		}
	}
}