46. Permutations

class Solution {
  public void backtrack(int n, ArrayList<Integer> nums, List<List<Integer>> output, int first) {
    // if all integers are used up
    if (first == n)
      output.add(new ArrayList<Integer>(nums));
    for (int i = first; i < n; i++) {
      // place i-th integer first 
      // in the current permutation
      Collections.swap(nums, first, i);
      // use next integers to complete the permutations
      backtrack(n, nums, output, first + 1);
      // backtrack
      Collections.swap(nums, first, i);
    }
  }

  public List<List<Integer>> permute(int[] nums) {
    // init output list
    List<List<Integer>> output = new LinkedList();

    // convert nums into list since the output is a list of lists
    ArrayList<Integer> nums_lst = new ArrayList<Integer>();
    for (int num : nums)
      nums_lst.add(num);

    int n = nums.length;
    backtrack(n, nums_lst, output, 0);
    return output;
  }
}

코드 직접 짜보려고 했는데..시간이 없네요 ㅎ
그리고 해본거 개망함^^