[ZOJ 1006] Do the Untwist (模拟实现解密)

题目链接：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemId=6

题目大意：给你加密方式，请你求出解密。

直接逆运算搞，用到同余定理

 #include <cstdio>
 #include <cstdlib>
 #include <string>
 #include <iostream>
 #include <cstring>
 #include <algorithm>
 #include <cctype>
 #include <vector>
 #include <map>
 #include <set>
 #include <iterator>
 #include <functional>
 #include <cmath>
 #include <numeric>
 using namespace std;
 typedef long long LL;
 typedef pair<int,int> PII;
 typedef vector<int> VI;
 #define PB push_back
 #define MP make_pair
 #define SZ size()
 #define CL clear()
 #define AA first
 #define BB second
 #define EPS 1e-8
 #define ZERO(x) memset((x),0,sizeof(x))
 const int INF = ~0U>>;
 const double PI = acos(-1.0);

 int get_num(char c){
     if( c=='_' ) return ;
     if( c=='.') return ;
     return c-'a'+;
 }

 char get_char(int n){
     if( n== ) return '_';
     if( n== ) return '.';
     return n+'a'-;
 }

 int main(){
     int k;
     while( scanf("%d",&k),k ){
         char buff[];
         scanf("%s",buff);
         int plaincode[],ciphercode[];
         ZERO(plaincode); ZERO(ciphercode);
         int n = strlen(buff);
         for(int i=;i<n;i++){
             ciphercode[i] = get_num(buff[i]);
         }
         for(int i=;i<n;i++){
             plaincode[(k*i)%n] = (ciphercode[i] + i)%;
         }
         for(int i=;i<n;i++){
             putchar(get_char(plaincode[i]));
         }
         puts("");
     }
     return ;
 }