【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].

public class Solution {

    public ArrayList<ArrayList<Integer>> permute(int[] num) {

        ArrayList<ArrayList<Integer>> re =new ArrayList<ArrayList<Integer>>();

        if(num.length==0)

            return re;

        ArrayList<Integer> ai = new ArrayList<Integer>();

        ArrayList<ArrayList<Integer>> te =new ArrayList<ArrayList<Integer>>();

        ai.add(num[0]);

        re.add(ai);

        te.add(ai);

        if(num.length==1)

            return re;

        for(int i=1;i<num.length;i++){

            int t = num[i];

            Iterator<ArrayList<Integer>> it = re.iterator();

            te =new ArrayList<ArrayList<Integer>>();

            while(it.hasNext()){

                ai = it.next();

                for(int j=0;j<ai.size();j++){

                    ai.add(j, t);

                    ArrayList<Integer> tem = new ArrayList<Integer>();

                    for(int mm=0;mm<ai.size();mm++)

                        tem.add(ai.get(mm));

                    te.add(tem);

                    ai.remove(j);

                }

                ArrayList<Integer> ttem = new ArrayList<Integer>();

                for(int nn=0;nn<ai.size();nn++)

                    ttem.add(ai.get(nn));

                ttem.add(t);

                te.add(ttem);

}

            re=te;

            te=new ArrayList<ArrayList<Integer>>();

        }

        return re;
}

}

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