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],
]

解法：采用递归的方法，运用dfs来求解这个问题

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new ArrayList<>();
        if(n <= 0 || k > n)
            return res;
        List<Integer> list = new ArrayList<>();
        dfs(n,k,1,list,res);
        return res;
    }
    public void dfs(int n, int k, int start, List<Integer> list, List<List<Integer>> res){
        if(list.size() == k) {
            res.add(new ArrayList<>(list));
            return;
        }
        for(int i = start; i <= n; i++){
            list.add(i);  //添加一个新的元素
            dfs(n,k,i+1,list,res);
            list.remove(list.size()-1); // 删除刚才添加进去的元素
        }
    }
}