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].

public class Solution {
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        int len = num.length;
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(len<=0) return res;
        boolean [] visited = new boolean[len];
        get(res,new ArrayList<Integer>(),0,num,visited);
        return res;
    }
    public void get(ArrayList<ArrayList<Integer>> res,ArrayList<Integer>output,int depth,int []num,boolean []visited){
        if(depth==num.length){
            ArrayList<Integer> temp = new ArrayList<Integer>();
            temp.addAll(output);
            res.add(temp);
            return;
        }
        for(int i=0;i<num.length;i++){
            if(!visited[i]){
                visited[i]=true;
                output.add(num[i]);
                get(res,output,depth+1,num,visited);
                output.remove(output.size()-1);
                visited[i] = false;
            }
        }
    }
}