int[] a = {1,2,3,4}; int[] b = {3,4,5,6}; 交集：2个元素重叠的部分.a∩b = {3,4}; 并集：2个元素合并.a∪b = {1,2,3,4,5,6}; 差集：减去重叠的部分.a?b = {1,2,5,6};