Leetcode:Combinations 生成组合

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],
]

 

题解分析：

典型的dfs

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int>> result;
        if (n == 0) return result;
        vector<int> path;
        dfs(n, k, 1, 0, path, result);
        return result;
    }
    
    void dfs(int n, int k, int start, int cur, vector<int>& path, vector<vector<int>>& result) {
        if (cur == k) {
            result.push_back(path);
        }
    
        for (int i = start; i <= n; ++i) {
            path.push_back(i);
            dfs(n, k, i + 1, cur + 1, path, result);
            path.pop_back();
        }
    }

};