public class Permutations {

public static void main(String[] args) {
int[] nums = {1, 2, 3};

List<List<Integer>> ans = permute(nums);

System.out.println(ans.toString());
}

public static List<List<Integer>> permute(int[] nums) {
List<List<Integer>> ans = new ArrayList<>();
if (nums == null || nums.length == 0) {
return ans;
}

recurse(nums, ans, 0);

return ans;
}

private static void recurse(int[] nums, List<List<Integer>> ans, int depth) {
int len = nums.length;

// Reach the end of nums, add this solution to answer
if (depth == len - 1) {
List<Integer> solution = new ArrayList<>();

for (int n : nums) {
}

return;
}

// For depth = 0, this first slot could be len situations from num[0] to num[len-1]
// Once the first slot determined, continue to go through rest elemennts

// The reason why starts from depth instead of depth+1 is that element remaining at same
// slot is also one situation
for (int j = depth; j < len; j++) {
swap(nums, depth, j);

recurse(nums, ans, depth + 1);

swap(nums, depth, j);
}
}

static void swap(int[] nums, int x, int y) {
int tmp = nums[x];
nums[x] = nums[y];
nums[y] = tmp;
}
}