LeetCode（26）-Binary Tree Level Order Traversal II

题目：

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]
]

思路：

代码：

/**
* 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>> levelOrderBottom(TreeNode root) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
levelOrderBottomHelper(root, result, 1);
return result;
}

public void levelOrderBottomHelper(TreeNode root, List<List<Integer>> result, int depth) {
if (root == null)
return;
List<Integer> list;
if (result.size() < depth) {
list = new ArrayList<Integer>();
result.add(0, list);
} else {
list = result.get(result.size() - depth);
}
list.add(root.val);
levelOrderBottomHelper(root.left, result, depth+1);
levelOrderBottomHelper(root.right, result, depth+1);
}
}