"Trochu" efektívnejšia verzia:

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

int main() {
	char number[11], buffer[6];
	int len, size, index, left, leftRev, right;

	printf("Zadaj cislo (max. 10 cifier): ");
	fgets(number, 11, stdin);
	len = strlen(number);
	if (number[len - 1] == '\n')
		number[--len] = 0;
	size = (len + 1) / 2;
	index = len / 2;
	buffer[size] = 0;
	strncpy(buffer, number + index, size);
	right = atoi(buffer);
	strncpy(buffer, number, size);
	left = atoi(buffer);
	strrev(buffer);
	leftRev = atoi(buffer);
	if (right >= leftRev) {
		left++;
		itoa(left, number, 10);
		itoa(left, number + index, 10);
		strrev(number + index);
	} else
		strncpy(number + index, buffer, size);
	printf("%s\n", number);
	return 0;
}