题意:给你一个字符串,求打乱字符后,有多少种回文串。 (题于文末)
知识点:
n个元素,其中a1,a2,····,an互不相同,进行全排列,可得n!个不同的排列。
若其中某一元素ai重复了ni次,全排列出来必有重复元素,其中真正不同的排列数应为 ,即其重复度为ni!
同理a1重复了n1次,a2重复了n2次,····,ak重复了nk次,n1+n2+····+nk=n。
题解:
字符串长度为len,若每个元素的重复次数ni为奇数的个数count>1,则无法形成回文串。
当能构成回文串时,我们只需考虑这个回文串左半部分的情况,所以这个问题也就变成了求一半字符串的有重复的全排列。
因为涉及到除法取模的问题,所以用到逆元。
逆元可以用扩展欧几里得,或费马小定理。/*6.逆元*/
#include<iostream> #include<cstdio> #include<cstring> using namespace std; typedef long long LL; const LL MOD=1e9+7; int cnt[260]; char ch[1005]; LL jiecheng(int n) { if(n==0) return 1; LL ans=1; for(int i=1;i<=n;i++) ans=ans*i%MOD; return ans; } LL x,y; LL gcd(LL a,LL b) { LL t,d; if(b==0) { x=1,y=0; return a; } d=gcd(b,a%b); t=x, x=y, y=t-(a/b)*y; return d; } int main() { int t; cin>>t; while(t--) { memset(cnt,0,sizeof(cnt)); scanf("%s",ch); int len=strlen(ch); for(int i=0;i<len;i++) { cnt[ch[i]-‘ ‘]++; } int count=0; for(int i=0;i<260;i++) { if(cnt[i]&1) count++; cnt[i]/=2; } if(count>1) { cout<<0<<endl; continue; } LL ans=jiecheng(len/2)%MOD; for(int i=0;i<260;i++) { if(cnt[i]>0) { gcd(jiecheng(cnt[i]),MOD); if(x<0) x+=MOD; ans=ans*x%MOD; } } cout<<ans<<endl; } }
xiaoxin juju needs help
Time Limit:1000MS Memory Limit:65536KB 64bit IO Format:%I64d & %I64u
Description
As we all known, xiaoxin is a brilliant coder. He knew **palindromic** strings when he was only a six grade student at elementry school.
This summer he was working at Tencent as an intern. One day his leader came to ask xiaoxin for help. His leader gave him a string and he wanted xiaoxin to generate palindromic strings for him. Once xiaoxin generates a different palindromic string, his leader will give him a watermelon candy. The problem is how many candies xiaoxin‘s leader needs to buy?
Input
This problem has multi test cases. First line contains a single integer $T(T\leq 20)$ which represents the number of test cases.
For each test case, there is a single line containing a string $S(1 \leq length(S) \leq 1,000)$.
Output
For each test case, print an integer which is the number of watermelon candies xiaoxin‘s leader needs to buy after mod $1,000,000,007$.
Sample Input
3 aa aabb a
Sample Output
1 2 1