程序代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MIN(a,b) (a>b?b:a)
#define MAXCHAR 10000
int main (int argc,char **argv)
{
    char *c = (char *)malloc(MAXCHAR*sizeof(char));
    char *bestchar;
    int bestmatch[2]={0,0};
    int beststep,step,maxstep;
    int i,j;
    int len;

    printf("Pleas input the string:\n");
    gets(c);
    len = strlen(c);

    //get best match
    for (i=0;i<len ;i++ )
        for (j=i+1;j<len ;j++ )
        {
            if (c[i] == c[j])
            {
                step = 1;
                maxstep = MIN(len-i,len-j);

                while(step<maxstep && c[i+step]==c[j+step]) step++;
   
                if (bestmatch[1]<step)
                {
                    bestmatch[0] = i;
                    bestmatch[1] = step;
                }
            }
        }

    //copy best match to bestchar
    bestchar = (char *)malloc(bestmatch[1]*sizeof(char));
    for (i=0;i<bestmatch[1] ;i++ ) bestchar[i] = c[i+bestmatch[0]];
    bestchar[i]='\0';

    printf("The longest copied string is %s\n",bestchar);

    free(bestchar);
    free(c);
}