[LeetCode] 46. Permutations

Given a collection of distinct 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 class Solution {
    
    private void swap(Integer[] nums, int i, int j){
        
        // Error: Note, i mustn't = j, or it clear the integer with bitwise method.
        if(i == j)
            return;
        
        nums[i] = nums[i]^nums[j];
        nums[j] = nums[i]^nums[j];
        nums[i] = nums[i]^nums[j];
        return;
        
    }
    
    public void permuteAux(Integer[] nums, int index, List> ret){
        
        int len = nums.length;
        
        if(index == len){
            ArrayList t = new ArrayList(Arrays.asList(nums));
            ret.add(t);
            return;
        }
        

        
        for(int i = index; i < len; ++i){
            swap(nums, index, i);
            permuteAux(nums, index+1, ret);
            swap(nums, index, i);
        }
        
    }
    
    
    public List> permute(int[] nums) {
        
        
        List> ret = new ArrayList>();
        
        int len = nums.length;
        
        if(0 == len)
            return ret;
            
        Integer[] iNums = new Integer[nums.length];
        
        for(int i = 0; i < len; ++i){
            iNums[i] = nums[i];
        }
        
        permuteAux(iNums, 0, ret);
            
        return ret;
        
    }
}