107. Binary Tree Level Order Traversal II

题目描述【Leetcode】

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] ]

这道题就是从下到上输出每个高度的节点的值，我的做法是先求出总的深度，在根据深度来依次存放：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */int dep(TreeNode*root){    if(!root) return 0;    int l = dep(root->left);    int r = dep(root->right);    return 1+max(l,r);}void f(TreeNode*root, vector<vector<int>>&re,int h){    if(!root) return;    if(h >= 0){    re[h].push_back(root->val);    f(root->left,re,h-1);    f(root->right,re,h-1);    }}class Solution {public:    vector<vector<int>> levelOrderBottom(TreeNode* root) {        int d = dep(root);        vector<vector<int>>re;        re.resize(d);        f(root,re,d-1);        return re;    }};