Leetcode 107. Binary Tree Level Order Traversal II 二叉树层次遍历2 解题报告

1 解题思想

2 原题

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]
]

3 AC解

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*
* 先序遍历，使用index标识当前的深度，放入对应的List当中就可以了
*/
public class Solution {
List<List<Integer>> result;
public void dfs(int index,TreeNode root){
if(root==null)
return ;
if(result.size()<=index){
result.add(new ArrayList<Integer>());
}
result.get(index).add(root.val);
dfs(index+1,root.left);
dfs(index+1,root.right);
}
public List<List<Integer>> levelOrder(TreeNode root) {
result=new ArrayList<List<Integer>>();
dfs(0,root);
return result;
}
}