题目描述：
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]
]
输入一棵二叉树，从底层叶子节点开始，将每层的节点存成一个list，然后逐层将每个list存入结果集中。
思路：
由于逐层遍历，显然是BFS，不过最底层的叶子集合需要放在最上，每次添加结果集时，将当前层的节点集合插入在首位即可。
实现代码：/**
* Definition for a binary tree node.
* public class TreeNode {
* public int val;
* public TreeNode left;
* public TreeNode right;
* public TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public IList<IList<int>> LevelOrderBottom(TreeNode root)
{
var result = new List<IList<int>>();
if(root == null){
return result;
}
var nodes = new List<TreeNode>(){root};
Bfs(nodes, ref result);
return result;
}
private void Bfs(List<TreeNode> parents, ref List<IList<int>> result)
{
if(parents.Count == 0){
return ;
}
var children = new List<TreeNode>();
var nodes = new List<int>();
for(var i = 0;i < parents.Count; i++){
nodes.Add(parents[i].val);
LoadChildren(parents[i], ref children);
}
result.Insert(0, nodes);
Bfs(children, ref result);
}
private void LoadChildren(TreeNode node , ref List<TreeNode> nodes)
{
if(node.left != null){
nodes.Add(node.left);
}
if(node.right != null){
nodes.Add(node.right);
}
}
}