c - find palindrome - c

Question

I'm a newcomer to programming, and I chose C as my first language(been learning it for a month or so).
I've been trying to solve this palindrome question for hours and still couldn't come up with a satisfying solution.
The question is here (from SPOJ), and here's my code:
#include <stdio.h>
#include <string.h>
void plus_one(char *number);
int main(void)
{
char number[1000001];
int i, j, m, k, indicator;
int a;
scanf("%d", &j);
for (i = 0; i < j; i++) {
scanf("%s", number);
k = 1;
while (k != 0) {
plus_one(number);
a = strlen(number);
indicator = 1;
for (m = 0; m < a / 2; m++) {
if (number[m] != number[a - m - 1]) {
indicator = 0;
break;
}
}
if (indicator != 0) {
printf("%s\n", number);
k = 0;
}
}
}
return 0;
}
void plus_one(char *number)
{
int a = strlen(number);
int i;
number[a - 1]++;
for (i = a; i >= 0; i--){
if (number[i - 1] == ':') {
number[i - 1] = '0';
number[i - 2]++;
}
else
break;
}
if (number[0] == '0') {
number[0] = '1';
strcat(number, "0");
}
return;
}
My idea was to examine every number greater than the input until a palindrome is found, and it worked well on my computer. But SPOJ responded "time limit exceeded", so I guess I need to find the next palindrome possible myself instead of using brute force. Can someone please give me a hint about how I can make this go faster? Thanks!

Answer 1

Since you're asking for a hint and not for C code (which I'm notoriously bad at), here's what I would do:
Determine if the number k has an even or odd number of digits, store that in a boolean called odd.
Take the first half of the number k, including the middle digit if odd is true, and store it in a variable called half.
808 -> 80
2133 -> 21
Mirror the half variable, taking care to not duplicate the middle digit if odd is true, and store it in a variable called mirror.
80 -> 808
21 -> 2112
Check if mirror > k
If true: you found your result
If false: increment half and start over from step 3.
(After maximum one increment you're guaranteed to have found your result.)
80 -> 81 -> 818
21 -> 22 -> 2222
Here's a JavaScript implementation for your reference:
const palin = (k) => {
const mirror = (half, odd) => half + Array.from(half).reverse().join('').substring(odd);
const s = k.toString();
const odd = s.length % 2;
const half = s.substring(0, Math.floor(s.length / 2) + odd);
let mirrored = mirror(half, odd);
if (+mirrored <= k) {
mirrored = mirror((+half + 1).toString(), odd);
}
return mirrored;
}
console.log(palin(5));
console.log(palin(808));
console.log(palin(2133));

Answer 2

Welcome to the site. What you have posted is commendable for someone who has only been using C for a month! Anyway ... I think your suspicion is correct. Using 'brute force' to find the next palindrome is probably the not to way go.
This question is as much about algorithm design as about C. Nonetheless, how you handle char[] representations of integers in C is interesting and relevant. FWIW, my attempt is pasted below.
It accepts a char[] representation of the number (n) and the number of digits (k) as arguments, and returns 1 on success or 0 on failure (another pass needed).
static int next_palindrome(char *n, size_t k) {
unsigned i = 0, carry = 0;
char tmp = 0;
int finished = 1;
for (i = 0; i < k; i++) {
if (carry) {
finished = 0;
*(n + k - i - 1) = *(n + k - i - 1) + 1;
if (*(n + k - i - 1) == 10) {
*(n + k - i - 1) = 0;
carry = 1;
} else
carry = 0;
continue;
}
if (i >= k / 2) continue;
if (*(n + k - i - 1) == *(n + i)) continue;
tmp = *(n + k - i - 1);
*(n + k - i - 1) = *(n + i);
if (tmp > *(n + i)) {
carry = 1;
}
}
return finished;
}
I have only tested it on numbers with < 64 digits so far, but have no reason to believe it will fail for larger numbers of digits.
Sample usage: http://codepad.org/3yyI9wEl