全排列 II

给定一个可包含重复数字的序列 nums ，按任意顺序 返回所有不重复的全排列。

示例 1：

输入：nums = [1,1,2]
输出：[[1,1,2], [1,2,1], [2,1,1]]

示例 2：

输入：nums = [1,2,3]
输出：[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

提示：

• 1 <= nums.length <= 8
• -10 <= nums[i] <= 10

class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    public List<List<Integer>> permuteUnique(int[] nums) {
        dfs(nums, 0);
        return ans;
    }
    private void dfs(int[] nums, int cur) {
        if (cur == nums.length) {
            List<Integer> line = new ArrayList<>();
            for (int i : nums) {
                line.add(i);
            }
            ans.add(line);
        } else {
            for (int i = cur; i < nums.length; i++) {
                if (canSwap(nums, cur, i)) {
                    swap(nums, cur, i);
                    dfs(nums, cur + 1);
                    swap(nums, cur, i);
                }
            }
        }
    }
    private boolean canSwap(int nums[], int begin, int end) {
        for (int i = begin; i < end; i++) {
            if (nums[i] == nums[end]) {
                return false;
            }
        }
        return true;
    }
    private void swap(int nums[], int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}