77. 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],
]
##思路
本题是一个典型的组合问题，第一种思路是递归的思想，代码如下：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> tmp;
        helper(n, k, 1, tmp, res);
        return res;
    }
    void helper(int n, int k, int level, vector<int>& tmp, vector<vector<int>>& res) {
        if (tmp.size() == k) res.push_back(tmp);
        for (int i = level; i <= n; ++i) {
            tmp.push_back(i);
            helper(n, k, i + 1, tmp, res);
            tmp.pop_back();
        }
    }
};

第二种是官方的解法，很巧妙，如下：

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