程式碼如下：

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

void getCommon(char str1[], char str2[], char * str3);
int stringLength(char * str);

void main(){
	char str1[30];
	char str2[30];
	char str3[30];
	fgets(str1, sizeof(str1), stdin);
	fgets(str2, sizeof(str2), stdin);
	getCommon(str1, str2, str3);
	printf("%s\n", str3);
}
int stringLength(char * str){
	int len = 0;
	while (*str != '\0'){
		len++;
		str++;
	}
	return len;
}
void getCommon(char str1[], char str2[], char * str3){
	int len1, len2;
	int i, j, k;
	int max = 0;
	int start = -1;

	len1 = stringLength(str1);
	len2 = stringLength(str2);
	for (i = 0; i < len1; i++){
		for (j = 0; j < len2; j++){
			if (str1[i] == str2[j]){
				for (k = 0; (str1[i + k] == str2[j + k] && str1[i + k] != '\0'); k++);
				if (max < k){
					max = k;
					start = i;
				}
			}
		}
	}

	if (start == -1){
		str3[0] = '\0';;
	}
	else{
		memcpy(str3, &str1[start], max);
		str3[max] = '\0';
	}
}