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Java Solution for LeetCode algorithm problems, continually updating

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package g0001_0100.s0047_permutations_ii;

// #Medium #Array #Backtracking #Algorithm_II_Day_10_Recursion_Backtracking
// #2022_06_16_Time_2_ms_(93.29%)_Space_48_MB_(59.58%)

import java.util.ArrayList;
import java.util.List;

public class Solution {
    private List> ans;

    public List> permuteUnique(int[] nums) {
        ans = new ArrayList<>();
        permute(nums, 0);
        return ans;
    }

    private void permute(int[] nums, int p) {
        if (p >= nums.length - 1) {
            List t = new ArrayList<>(nums.length);
            for (int n : nums) {
                t.add(n);
            }
            ans.add(t);
            return;
        }
        permute(nums, p + 1);
        boolean[] used = new boolean[30];
        for (int i = p + 1; i < nums.length; i++) {
            if (nums[i] != nums[p] && !used[10 + nums[i]]) {
                used[10 + nums[i]] = true;
                swap(nums, p, i);
                permute(nums, p + 1);
                swap(nums, p, i);
            }
        }
    }

    private void swap(int[] nums, int i, int j) {
        int t = nums[i];
        nums[i] = nums[j];
        nums[j] = t;
    }
}