>>165 正攻法でやってみた。
#include <stdio.h>
void myitoa(int n, char s[])
{
int i, j, count = 0, sign;
char c;
if ((sign = n) < 0) n = -n;
i = 0;
do {
switch (n % 10) {
case 0: case 1: case 2: case 3: case 4:
case 5: case 6: case 7: case 8: case 9:
s[i++] = '0' + (n % 10); break;
}
} while (count++, (n /= 10) > 0);
if (sign < 0) s[i++] = '-';
s[i] = '\0';
for (i = 0, j = count - 1; i < j; i++, j--) c = s[i], s[i] = s[j], s[j] = c;
}

int main(void)
{
char s[12];
int x, n;
printf("10桁までの任意の数字を入力=", x);
scanf("%d", &x);
myitoa(x, s);
printf("何桁目を表示しますか？");
scanf("%d", &n);
　printf("%d桁目の数字は%cです\n", n, s[n - 1]);
return 0;
}