combine

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Combinations

Total Accepted: 14673 Total Submissions: 48919 My Submissions

Combinations

Total Accepted: 14673 Total Submissions: 48919 My Submissions

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

[Thoughts]
Similar as "Conbination Sum". But here the terminate condition is "k", not sum.

[Code]

class Solution {
public:
vector<vector<int> > combine(int n, int k) {
vector<vector <int>> res;
vector<int> solution;
if (n<=0)
return res;

newcombine(n, k, 1, res, solution);
return res;
}

void newcombine(int n, int k, int level, vector<vector<int>> &res, vector<int> &solution){
if (solution.size()==k){ //这个事终止条件，达到K则返回
res.push_back(solution);
return;
}

for (int i=level; i<=n; i++){ //每次循环是从Level开始的，不是从i
solution.push_back(i);
newcombine(n, k, i+1, res, solution);
solution.pop_back(); //这个pop_back是把最后一个元素pop出来，从而达到值变最后一个数的目的
}
}
};