int x=10; x += x -= x-x;
A. 30 B. 20 C. 40 D. 10
x += x -= x - x; 等价于 x = x + (x -= x - x); 等价于 x = x + (x - (x - x)); 等价于 x = 10 + 10 - (10 - 10);
答案是20,选B