Fair Candy Swap

给两个整数组, 交换一对整数, 让两个整数组想加的和相等. 首先先求两个数组的和, 然后求他们的差, 这个差值就是交换两个数的差值.

class Solution {
    public int[] fairCandySwap(int[] A, int[] B) {
        int sumA = 0;
        int sumB = 0;
        for(int a : A)
            sumA += a;
        for(int b : B)
            sumB += b;
        int diff = (sumB - sumA) / 2;
        Set<Integer> set = new HashSet<Integer>();
        for(int i = 0 ; i < B.length; i++) {
            set.add(B[i]);
        }
        for(int a : A) {
            if(set.contains(a+diff))
                return new int[]{a, a+diff};
        }
        return null;
    }
}