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;

    }
};