#include<stdio.h>
int main()
{unsigned int a = -1;
printf("%d\n",a);
return 0;
}

因为%d

-1是返回正确答案有疑问，a应该是4294967295。
unsigned int a = -1;  可以看成是 unsigned int a = (unsigned int)(-1);
printf("%d\n",a);  可以看成是 printf("%d\n",(int)a);

unsigned是代表无符号，unsigned int的意思是只能是正整数不能等于-1