有以下程序 #include struct tt { int x; struct tt *y; } *p; struct tt a[4]={20,a+1,15,a+2,30,a+3,17,a}; main() { int i; p=a; for(i=1; i<=2; i++) { printf("%d,", p->x ); p=p->y; } } 程序的运行结果是