修改itoa函数，使得该函数可以接受三个参数。其中第三个参数为最小字符宽度。

#include <stdio.h>
#include <limits.h>

void itoa(int n, char s[], int width);
void reverse(char s[]);

int main(void) {
char buffer[20];

itoa(INT_MIN, buffer, 7);
printf("Buffer:%s\n", buffer);

return 0;
}

void itoa(int n, char s[], int width) {
int i, sign;

if ((sign = n) < 0)
n = -n;
i = 0;
do {
s[i++] = n % 10 + '0';
printf("%d %% %d + '0' = %d\n", n, 10, s[i-1]);
} while ((n /= 10) > 0);
if (sign < 0)
s[i++] = '-';

while (i < width )    /*  Only addition to original function  */
s[i++] = ' ';

s[i] = '\0';
reverse(s);
}

void reverse(char s[]) {
int c, i, j;
for ( i = 0, j = strlen(s)-1; i < j; i++, j--) {
c = s[i];
s[i] = s[j];
s[j] = c;
}
}