最长双回文串（模板+dp）

题目链接

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 inline ll read(){
     int x = , f = ; char ch = getchar();
     while(ch > '' || ch < ''){if (ch == '-') f = -; ch = getchar();}
     while(ch >= '' && ch <= ''){ x = x*+ch-''; ch = getchar();}
     return x*f;
 }

 /************************************************************************/

 const int maxn = 1e6+;
 char s[maxn];
 char str[maxn];
 int dp1[maxn], p[maxn];
 int dp2[maxn];
 int len;

 void manacher(char s[], int len){
     int ans = ;
     for(int i = ;i <= len;i++){
         str[i<<] = s[i];
         str[(i<<)+] = '#';
     }
     str[] = '#'; str[len*+] = '#';
     str[] = '&'; str[len*+] = '$';
     len = len*+;
     int j = , k;
     for(int i = ;i <= len;){
         while(str[i-j-] == str[i+j+]) j++;
         p[i] = j;
         if(j > ans) ans = j;
         for(k = ;k <= j && (p[i]-k != p[i-k]);k++){
             p[i+k] = min(p[i-k], p[i] - k);
         }
         i += k;
         j = max(j-k, );
     }
 }

 int main(){
     scanf("%s", s+);
     len = strlen(s+);
     manacher(s, len);
     len = len*+;
     cout << "str: " << str << endl;
     for(int i = ;i <= len;i++){
         for(int j = p[i];j >= ;j--){
             if(dp1[i+j] >= j) break;
             dp1[i+j] = j;
         }
         for(int j = p[i];j >= ;j--){
             if(dp2[i-j] >= j) break;
             dp2[i-j] = j;
         }
     }
     int ans = ;
     for(int i = ;i <= len-;i++){
         if(dp1[i] && dp2[i])
             ans = max(ans, dp1[i] + dp2[i]);
     }
     cout << ans << endl;
     return ;
 }