关键点：
1.反转
2.竖着判断，慢慢增加最长前缀的长度

输入样例

3
Itai nyan~
Ninjin wa iyadanyan~
uhhh nyan~

输出样例

nyan~

C++代码

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int N = 310;
int n, minlen = 256, ans = 0;
char s[100][256];
int main( ){
    scanf("%d", &n);
    getchar();
    for ( int i=0; i < n; i++ ) {
        cin.getline(s[i], N);
        int len = strlen(s[i]);
        if ( len < minlen ) minlen = len;
        for ( int j=0; j < len / 2; j++ ) {
            char tmp = s[i][j];
            s[i][j] = s[i][len - j -1];
            s[i][len -j - 1] = tmp;
        }
    }
    for ( int i=0; i < minlen; i++ ) {
        char c = s[0][i];
        bool same = true;
        for ( int j=1; j <n; j++ ) {
            if ( c != s[j][i] ) {
                same = false;
                break;
            }
        }
        if( same ) ans++;
        else break;
    }
    if ( ans ) {
        for ( int i = ans-1; i>=0; i-- ) {
            printf("%c", s[0][i]);
        }
    }
    else {
        printf("nai");
    }
    return 0;
}