LeetCode | Permutations II

题目

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,

[1,1,2]

 have the following unique permutations:

[1,1,2]

, 

[1,2,1]

,
and 

[2,1,1]

.
分析
有重复数字的全排列也是所有数字轮流排第一，剩余再全排列，需要注意相同数字只有一次机会排到第一。

代码

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;

public class PermutationsII {
private ArrayList<ArrayList<Integer>> result;
private int N;
private ArrayList<Integer> list;

public ArrayList<ArrayList<Integer>> permuteUnique(int[] num) {
result = new ArrayList<ArrayList<Integer>>();
N = num.length;
list = new ArrayList<Integer>();
for (int i = 0; i < N; ++i) {
list.add(num[i]);
}
solve(0);
return result;
}

private void solve(int k) {
if (k >= N) {
result.add(new ArrayList<Integer>(list));
return;
}
Set<Integer> set = new HashSet<Integer>();
for (int i = k; i < N; ++i) {
int value = list.get(i);
if (!set.contains(value)) {
swap(i, k);
solve(k + 1);
swap(i, k);
set.add(value);
}
}
}

private void swap(int i, int j) {
int temp = list.get(i);
list.set(i, list.get(j));
list.set(j, temp);
}
}