# WA in SPOJ EPALIN

hi! can anyone help me. i got WA in http://www.spoj.com/problems/EPALIN/
i have tried and optimised as much as i could.
i tried it with these test cases:-
abcdefglananahijklmnopqrstananal
laxalghijklaxal
jklaxalghijklaxal

any help would be appreciated (any different test case or hint)

CODE:-

@harindersingh ,
for input string : aaabcdxxxx , your output is : aaabcdxxxxdcbaaa i.e. printing an extra βxβ as correct output is : aaabcdxxxdcbaaa .

i think according to the problem statement characters can be only added and not deleted.
if it has to display aaabcdxxxdcbaaa, itβll have to delete 1 x from the end of original string

thanks harinder singh

Hello all.
Got Ac with this simplest implemenation for this problem

```
#include
using namespace std ;
#define ULL unsigned long long
#define MAXN 200015

char str[MAXN] ;
int N ;
ULL hashF[MAXN] , hashB[MAXN] , P , POW[MAXN];

void process(){

P = 37 ;
hashF[0] = 0 ;
hashB[N+1] = 0 ;
POW[0] = 1 ;
for(int i=1;i<=N;i++){
hashF[i] = hashF[i-1]*P + (str[i]) ;
POW[i] = POW[i-1]*P ;
}

for(int i=N;i>=1;i--){
hashB[i] = hashB[i+1]*P + (str[i]) ;
}
}

int main(){

while(scanf("%s",str+1) != EOF){

N = strlen(str+1) ;
process() ;
int len = N ;
for(int i=1;i<=N;i++){
ULL hash1 = hashF[N]-hashF[i-1]*POW[len] ;
ULL hash2 = hashB[i];
if(hash1 == hash2){
break ;
}
len -- ;
}
int need = N-len ;
str[need+N+1] = 0 ;
int low = 1 ;
int high = N+need ;
while(low <= high){
str[high] = str[low] ;
high -- ;
low ++ ;
}
printf("%s\n",str+1) ;
}
return 0 ;
}
```
1 Like

Hello ma5termind

But there is no surity as colloision may take place in this method. how to make sure that collision never occurs ?

Thanks