# Permutations

Given a collection of **distinct** integers, return all possible permutations.

**Example:**

```
Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]
```

```java
class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> list = new ArrayList<>();
        boolean[] used = new boolean[nums.length];
        backtrack(list, new ArrayList<>(), nums, used);
        return list;
    }

    private void backtrack(List<List<Integer>> list, List<Integer> tempList, int[] nums, boolean used[]) {
        if (tempList.size() == nums.length) {
            list.add(new ArrayList<>(tempList));
        } else {
            for (int i = 0; i < nums.length; i++) {
                if (used[i])
                    continue; // element already exists, skip
                used[i] = true;
                tempList.add(nums[i]);
                backtrack(list, tempList, nums, used);
                tempList.remove(tempList.size() - 1);
                used[i] = false;
            }
        }
    }
}
```
