LeetCode77 Combinations

题目：

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

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

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

分析：

求所有k长度的组合数。回溯的思路，每个元素可取可不取，到达k个元素则添加到结果中。

代码：

class Solution {
private:
    void helper(const vector<int>& seq, vector<int>& temp, int start, int k, vector<vector<int>>& result) {
        if (k > seq.size() - start) {
            return;
        }
        if (k == 0) {
            result.push_back(temp);
            return;
        }
        temp.push_back(seq[start]);
        helper(seq, temp, start + 1, k - 1, result);
        temp.pop_back();
        helper(seq, temp, start + 1, k, result);
        
    }
public:
    vector<vector<int>> combine(int n, int k) {
        vector<int> seq(n);
        vector<vector<int>> result;
        for (int i = 0; i < n; ++i) {
            seq[i] = i + 1;
        }
        vector<int> temp;
        helper(seq,temp,0,k,result);
        return result;
    }
};