【LeetCode】C# 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]

]

层序遍历，从底层遍历到根。

思路：跟上一题一样，最后在把结果翻过来。

/**
* 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 List<List<int>> LevelOrderBottom(TreeNode root) {
List<List<int>> res = new List<List<int>>();
List<List<int>> ret = new List<List<int>>();
wrtIn(ret,root,0);
foreach (List<int> item in ret)
res.Insert(0,item);
return res;
}
public void wrtIn(List<List<int>> res,TreeNode tree,int level){
if(tree == null) return;
if(level >= res.Count) res.Add(new List<int>());
res[level].Add(tree.val);
wrtIn(res,tree.left,level+1);
wrtIn(res,tree.right,level+1);
}
}