Combinations

题目：

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],]

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程序：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
int i = 0;
vector<int> p(k, 0);
while (i >= 0) {
p[i]++;
if (p[i] > n) --i;
else if (i == k - 1) result.push_back(p);
else {
   ++i;
   p[i] = p[i - 1];
}
}
return result;
    }
};