/in/foo.c: In function 'main':
/in/foo.c:3:5: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
     scanf("%d", &n); // 输入数组中元素的个数n
     ^~~~~
/in/foo.c:3:5: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:3:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
/in/foo.c:19:9: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
         printf("%d ", arr[i]);
         ^~~~~~
/in/foo.c:19:9: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:19:9: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:21:5: warning: incompatible implicit declaration of built-in function 'printf'
     printf("\n");
     ^~~~~~
/in/foo.c:21:5: note: include '<stdio.h>' or provide a declaration of 'printf'