最长双回文串（模板+dp）

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 inline ll read(){
 5     int x = 0, f = 1; char ch = getchar();
 6     while(ch > ‘9‘ || ch < ‘0‘){if (ch == ‘-‘) f = -1; ch = getchar();}
 7     while(ch >= ‘0‘ && ch <= ‘9‘){ x = x*10+ch-‘0‘; ch = getchar();}
 8     return x*f;
 9 }
10 
11 /************************************************************************/
12 
13 const int maxn = 1e6+7;
14 char s[maxn];
15 char str[maxn];
16 int dp1[maxn], p[maxn];
17 int dp2[maxn];
18 int len;
19 
20 void manacher(char s[], int len){
21     int ans = 0;
22     for(int i = 1;i <= len;i++){
23         str[i<<1] = s[i];
24         str[(i<<1)+1] = ‘#‘;
25     }
26     str[1] = ‘#‘; str[len*2+1] = ‘#‘;
27     str[0] = ‘&‘; str[len*2+2] = ‘$‘;
28     len = len*2+1;
29     int j = 0, k;
30     for(int i = 1;i <= len;){
31         while(str[i-j-1] == str[i+j+1]) j++;
32         p[i] = j; 
33         if(j > ans) ans = j;
34         for(k = 1;k <= j && (p[i]-k != p[i-k]);k++){
35             p[i+k] = min(p[i-k], p[i] - k);
36         }
37         i += k;
38         j = max(j-k, 0);
39     }
40 }
41 
42 int main(){
43     scanf("%s", s+1);
44     len = strlen(s+1);
45     manacher(s, len);
46     len = len*2+1;
47     cout << "str: " << str << endl;
48     for(int i = 1;i <= len;i++){
49         for(int j = p[i];j >= 1;j--){
50             if(dp1[i+j] >= j) break;
51             dp1[i+j] = j;
52         }
53         for(int j = p[i];j >= 1;j--){
54             if(dp2[i-j] >= j) break;
55             dp2[i-j] = j;
56         }
57     }
58     int ans = 0;
59     for(int i = 1;i <= len-2;i++){
60         if(dp1[i] && dp2[i])
61             ans = max(ans, dp1[i] + dp2[i]);
62     }
63     cout << ans << endl;
64     return 0;
65 }