LeetCode 46 - Permutations

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Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

 

public List> permute(int[] num) {
    List> result = new ArrayList<>();
    perm(result, num, 0);
    return result;
}

public void perm(List> result, int[] num, int pos) {
    if(pos == num.length) {
        List list = new ArrayList();
        for(int a:num) list.add(a);
        result.add(list);
        return;
    }
    for(int i=pos; i