【DFS】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],
]

解法一：递归DFS，在Subsets I的基础上限制个数
public class Solution {
public ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();

public void dfs(ArrayList<Integer> l, int n, int k, int pos){
if(l.size() > k) return;
if(l.size() == k){
res.add(new ArrayList(l));
return ;
}

for(int i=pos; i<=n; i++){
l.add(i);
dfs(l, n, k, i+1);
l.remove(l.size() - 1);
}
}

public ArrayList<ArrayList<Integer>> combine(int n, int k) {
ArrayList l = new ArrayList<Integer>();
dfs(l, n, k, 1);
return res;
}
}