UVA 12050 - Palindrome Numbers（数论+规律）

题目链接：12050 - Palindrome Numbers

#include <stdio.h>
#include <string.h>

int n, tab[105], tn, i, mi[105];

void table() {
int s = 9;
for (tn = 1; ; tn++) {
tab[tn] = tab[tn - 1] + s;
if (tn % 2 == 0) s *= 10;
if (tab[tn] == 1999999998) break;
}
mi[0] = 1;
for (int i = 1; i <= 10; i++)
mi[i] = mi[i - 1] * 10;
}

void f(int num) {
int ans[15], ansn = 0;
while (num) {
ans[ansn++] = num % 10;
num /= 10;
}
for (int i = 0; i < ansn; i++)
printf("%d", ans[i]);
printf("\n");
}

int main() {
table();
while (~scanf("%d", &n) && n) {
for (i = 1; i <= tn; i++) {
if (n <= tab[i]) break;
}
int num = n - tab[i - 1] + mi[(i - 1) / 2] - 1;
printf("%d", num);
if (i == 1) {
printf("\n");
continue;
}
if (i % 2) num /= 10;
f(num);
}
return 0;
}