LeetCode 254. Factor Combinations

Numbers can be regarded as product of its factors. For example,

8 = 2 x 2 x 2;
= 2 x 4.
Write a function that takes an integer n and return all possible combinations of its factors.

Note:

Each combination’s factors must be sorted ascending, for example: The factors of 2 and 6 is [2, 6], not [6, 2].
You may assume that n is always positive.
Factors should be greater than 1 and less than n.

Examples:
input: 1
output:

[]
input: 37
output:

[]
input: 12
output:

[
[2, 6],
[2, 2, 3],
[3, 4]
]
input: 32
output:

[
[2, 16],
[2, 2, 8],
[2, 2, 2, 4],
[2, 2, 2, 2, 2],
[2, 4, 4],
[4, 8]
]

思路：
1. 枚举所有的情况，用recursive比较合适。例如，32=2*16，把16继续分解！
2. 有一个trick，遍历i从2到i*i<=n，不用费劲做i

vector<vector<int>> getFactors(int n) {    vector<vector<int>> res;    vector<int> cur;    for(int i=2;i*i<=n;i++){        helper(res,cur,i,n);        }    return res;}void helper(vector<vector<int>>&res,vector<int> cur,int i,int n){    if(n%i) return;    int tmp=n/i;    cur.push_back(i);    cur.push_back(tmp);    res.push_back(cur);    cur.pop_back(tmp);//语法错误，应该是cur.pop_back();弹出最后一个，没有参数    for(int j=i;j*j<=tmp;j++){        helper(res,cur,j,tmp);      }}