Permutation

public class Solution {
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
        final int N = num.length;
        if (N > 0) {
            perm(num, N, 0, ans);
        }
        return ans;
    }
    
    private boolean containsDuplicate(int []num, int start, int end) {
        for (int i = start; i < end; i++) {
            if (num[i] == num[end]) {
                return true;
            }
        }
        return false;
    }
    
    public void perm(int []num, int N, int i, ArrayList<ArrayList<Integer>> ans) {
        if (i == N) {
            ArrayList<Integer> t = new ArrayList<Integer>();
            for (int n: num) {
                t.add(n);
            }
            ans.add(t);
        } else {
            for (int j = i; j < N; j++) {
                if (containsDuplicate(num, i, j)) {
                    continue;
                }
                swap(num, i, j);
                perm(num, N, i + 1, ans);
                swap(num, i, j);
            }
        }
    }
    
    private void swap(int []num, int i, int j) {
        int t = num[i];
        num[i] = num[j];
        num[j] = t;
    }
}