Array:Given an array of integers, every element appears twice except for one. Find that single one.

public int singleNumber(int[] nums) {
        if (nums==null||nums.length==0) {
            return 0;
        }
        
        Set set = new HashSet();
        for (int i = 0; i < nums.length; i++) {
            if (set.contains(nums[i])) {
                set.remove(nums[i]);
            } else {
                set.add(nums[i]);
            }
        }
        return Integer.valueOf(set.toArray()[0].toString());
    }