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PermutationsII.java

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import java.util.ArrayList;

/**

* 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].

*/

public class PermutationsII {

public ArrayList<ArrayList<Integer>> permuteUnique(int[] num) {

ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

permuteUnique(num, 0, result);

return result;

}

void permuteUnique(int[] num, int begin,

ArrayList<ArrayList<Integer>> result) {

if (begin > num.length - 1) {

ArrayList<Integer> item = new ArrayList<Integer>();

for (int h = 0; h < num.length; h++) {

item.add(num[h]);

}

result.add(item);

}

for (int end = begin; end < num.length; end++) {

if (isSwap(num, begin, end)) {

swap(num, begin, end);

permuteUnique(num, begin + 1, result);

swap(num, begin, end);

}

}

}

boolean isSwap(int[] arr, int i, int j) {

for (int k = i; k < j; k++) {

if (arr[k] == arr[j]) {

return false;

}

}

return true;

}

private void swap(int[] a, int i, int j) {

int tmp = a[i];

a[i] = a[j];

a[j] = tmp;

}

}