LeetCode: Combinations

LeetCode: Combinations

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],
]

地址：https://oj.leetcode.com/problems/combinations/
算法：递归搞定。$\binom{n}{k}=\binom{n-1}{k}+\binom{n-1}{k}$。代码：

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        if(n < 1 || k < 1)  return vector<vector<int> > ();
        return combineCore(n, k);
    }
    vector<vector<int> > combineCore(int n,int k){
        vector<vector<int> > result;
        if(k <= 0){
            result.push_back(vector<int>());
            return result;
        }
        if(n <= k){
            vector<int> temp;
            for(int i = 1; i <= n; ++i)
                temp.push_back(i);
            result.push_back(temp);
            return result;
        }
        result = combineCore(n-1, k-1);
        for(int j = 0; j < result.size(); ++j){
            result[j].push_back(n);
        }
        vector<vector<int> > temp = combineCore(n-1,k);
        result.insert(result.end(),temp.begin(),temp.end());
        return result;
    }
};