bzoj1692

题解：

二分最近的不相同

然后ｈａｓｈ判断是否相同

然后贪心

代码：

#include<bits/stdc++.h>
using namespace std;
#define ull unsigned long long
const int N=;
int n;
char s[N],s1[],ans[N];
ull h1[N],h2[N],pw[N];
int main()
{
    scanf("%d",&n);
    for (int i=;i<=n;i++)scanf("%s",s1+),s[i]=s1[];
    pw[]=;
    for (int i=;i<=n;i++)pw[i]=pw[i-]*;
    for (int i=;i<=n;i++)h1[i]=h1[i-]*+s[i];
    for (int i=n;i>=;i--)h2[i]=h2[i+]*+s[i];
    for (int i=,l1=,r1=n;i<=n;i++)
     {
        int l=,r=r1-l1+;
        while (l<=r)
         {
            int mid=(l+r)>>;
            if (h1[l1+mid-]-h1[l1-]*pw[mid]==h2[r1-mid+]-h2[r1+]*pw[mid])l=mid+;
            else r=mid-;
         }
        if (l>r1-l1+)l=;
        if (s[l1+l-]>s[r1-l+])ans[i]=s[r1],r1--;
        else ans[i]=s[l1],l1++;
     }
    for (int i=;i<=n;i++)
     {
        putchar(ans[i]);
        if (i%==)puts("");
     }
    return ;
}