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class Solution {
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  List<List<Integer>> res = new LinkedList<>();
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  public List<List<Integer>> permute(int[] nums) {
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    ArrayList<Integer> list = new ArrayList<>();
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    boolean[] visited = new boolean[nums.length];
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    backTrack(nums, list, visited);
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    return res;
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  }
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  private void backTrack(int[] nums, ArrayList<Integer> list, boolean[] visited) {
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    if (list.size() == nums.length) {
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      res.add(new ArrayList(list));
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      return;
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    }
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    for (int i = 0; i < nums.length; i++) {
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      if (!visited[i]) {
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        visited[i] = true;
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        list.add(nums[i]);
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        backTrack(nums, list, visited);
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        visited[i] = false;
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        list.remove(list.size() - 1);
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      }
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    }
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  }
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}