[Leetcode] Permutations

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Solution:

public class Solution {
    public List<List<Integer>> permute(int[] num) {
        List<List<Integer>> result=new ArrayList<List<Integer>>();
        List<Integer> al=new ArrayList<Integer>();
        boolean[] flag=new boolean[num.length];
        dfs(num,result,al,0,flag);
        return result;
    }

    private void dfs(int[] num, List<List<Integer>> result, List<Integer> al, int level,boolean[] flag) {
        // TODO Auto-generated method stub
        if(level>num.length)
            return;
        if(level==num.length){
            result.add(new ArrayList<Integer>(al));
            return;
        }
        for(int i=0;i<num.length;++i){
            if(!flag[i]){
                flag[i]=true;
                al.add(num[i]);
                dfs(num, result, al, level+1, flag);
                al.remove(al.size()-1);
                flag[i]=false;
            }
        }    
    }
}