euler4

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.
--------------------

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

void itoa(char *buf, int n)
{
    int i = 0;

    for (i=0; n>0; i++, n /= 10) {
        buf[i] = (n % 10) + '0';
    }

    buf[++i] = '\0';
}

int is_palindromic(int num)
{
    char buf[sizeof(num)*8];
    int len;
    int i;

    itoa(buf, num);
    len = strlen(buf);
    for (i=0; i<(len+1)/2; i++) {
        if(buf[i] != buf[len-i-1])
            return 0;
    }

    return 1;
}

int main(int argc, const char *argv[])
{
    int i, j;
    int result;
    int acc, mi, mj;

    for (i = 999; i > 100; i--) {
        for (j = i; j > 100; j--) {
            acc = i * j;
            if (is_palindromic(acc) ) {
                if (acc > result) {
                    result = acc;
                    mi = i, mj = j;

                    printf("result: %d, i %d, j %d\n", result, i, j);
                }
            }
        }
    }

    printf(" %d = %d x %d\n", result, mi, mj);

    return 0;
}

根据乘法交换律筛选了一些。再依据或然率可进一步优化。