77. Combinations (Recursion)

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：遍历n以内的正整数，每个数有放入与不放入两种选择=>带回溯的递归

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
        vector<int> pre;  
        dfs(result,pre,n,k,1);     
        return result;
    }
    
    void dfs( vector<vector<int>> &result, vector<int> pre, int n, int k, int depth)
    { 
        if(depth > n) return;
        pre.push_back(depth);

        if(pre.size() < k)
        {
            dfs(result,pre,n,k,depth+1); 
        }
        else
        {
            result.push_back(pre);
        }
        pre.pop_back();
        dfs(result,pre,n,k,depth+1);     
    }
};