Backtracking

BacktrackingPermutations.java

public List<List<Integer>> permute(int[] nums) {
    List<List<Integer>> ans = new ArrayList<>();
    backtrack(ans, new ArrayList<Integer>(), nums);
    return ans;
}
private void backtrack(List<List<Integer>> ans, List<Integer> ongoing, int[] nums) {
    if(ongoing.size() == nums.length) {
        ans.add(new ArrayList<>(ongoing));
    } else {
        for(int i = 0; i < nums.length; i++) {
            if(!ongoing.contains(nums[i])) {
                ongoing.add(nums[i]);
                backtrack(ans, ongoing, nums);
                ongoing.remove(ongoing.size() - 1);
            }
        }
    }
}