【LeetCode】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],
]

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        if(k>n)
            return null;
        ArrayList<Integer> ai = new ArrayList<Integer>();
        ArrayList<ArrayList<Integer>> re = new ArrayList<ArrayList<Integer>>();
        for(int m=0;m<n;m++){
            ai=new ArrayList<Integer>();
            ai.add(m+1);
            re.add(ai);
        }
        for(int i=1;i<k;i++){
            Iterator<ArrayList<Integer>> it = re.iterator();
            ArrayList<ArrayList<Integer>> tempre = new ArrayList<ArrayList<Integer>>();
            while(it.hasNext()){
                ArrayList<Integer> temp = it.next();
                int tt = temp.get(temp.size()-1);
                for(int j=tt+1;j<=n;j++){
                    ArrayList<Integer> newtemp = new ArrayList<Integer>();
                    newtemp.addAll(temp);
                    
                    newtemp.add(j);
                    tempre.add(newtemp);
                }
            }
            re=tempre;
        }
        return re;
        
    }
}