content: /**
   * File: permutations_ii.dart
   * Created Time: 2023-08-10
   * Author: liuyuxin (gvenusleo@gmail.com)
   */
  
  /* 回溯算法：全排列 II */
  void backtrack(
    List<int> state,
    List<int> choices,
    List<bool> selected,
    List<List<int>> res,
  ) {
    // 当状态长度等于元素数量时，记录解
    if (state.length == choices.length) {
      res.add(List.from(state));
      return;
    }
    // 遍历所有选择
    Set<int> duplicated = {};
    for (int i = 0; i < choices.length; i++) {
      int choice = choices[i];
      // 剪枝：不允许重复选择元素 且 不允许重复选择相等元素
      if (!selected[i] && !duplicated.contains(choice)) {
        // 尝试：做出选择，更新状态
        duplicated.add(choice); // 记录选择过的元素值
        selected[i] = true;
        state.add(choice);
        // 进行下一轮选择
        backtrack(state, choices, selected, res);
        // 回退：撤销选择，恢复到之前的状态
        selected[i] = false;
        state.removeLast();
      }
    }
  }
  
  /* 全排列 II */
  List<List<int>> permutationsII(List<int> nums) {
    List<List<int>> res = [];
    backtrack([], nums, List.filled(nums.length, false), res);
    return res;
  }
  
  /* Driver Code */
  void main() {
    List<int> nums = [1, 2, 2];
  
    List<List<int>> res = permutationsII(nums);
  
    print("输入数组 nums = $nums");
    print("所有排列 res = $res");
  }
  