HDU-4300-clairewd's message-KMP

Brief description: http://acm.hdu.edu.cn/showproblem.php? PID = 1, 4300

A table with 26 letters in one-to-one plaintext to ciphertext, and a string a with plaintext + ciphertext (with default value) are provided. The minimum length of A is obtained.

Analysis:

First, I tried to use brute force. Later I found that it was very time-consuming to judge the same strings. TLE had performed several times and finally found that KMP was acceptable. First, I set the first half of a (it must be plain text, this is obvious.) translate it into a secret, then obtain the next array of the new string, iterate the next array of the last letter (see the code for details), and keep moving to the length <= String Length/2 to stop, then we can find the complete a with the smallest length.

#include <cstdio>#define maxn 100010#include <cstring>int T, next[maxn];char table[30], ori[maxn], ans[maxn * 2], copy[maxn], mitoming[30];using namespace std;void get_next(char * pat){    memset(next, 0, sizeof(next));    int patlen = strlen(pat);    int p = -1, t = 0;    next[0] = -1;    while(t < patlen){        if(p == -1 || pat[t] == pat[p]){            t ++, p ++, next[t] = p; // (pat[t] == pat[p]) ? next[p] : p;        }else{            p = next[p];        }    }}int main(){    scanf("%d",&T);    while(T --){        scanf("%s %s", table, ori);        int len = strlen(ori);        int id = len; //        memset(copy,0,sizeof(copy));        memset(ans,0,sizeof(ans));        strcpy(copy, ori);        for(int i = 0; i < 26; i ++){            mitoming[table[i] - 'a'] = 'a' + i;        }        mitoming[26] = '\0';        for(int i = 0; i < len / 2; i ++){            copy[i] = mitoming[copy[i] - 'a'];        }//        printf("%s  be", copy);        get_next(copy);//        printf("%s af", copy);//        printf("%d kaka \n", strlen(copy));//        for(int i = 0; i < strlen(copy); i ++)//            printf("%d !! ", next[i]);//        puts("");        int i = len - 1, j;        while(next[i] >= len / 2){            i = next[i];        }//        printf("%d  be \n", i);        if(next[i] == 0)            id = len;        else            id = len - 1 - next[i];//        printf("%d\n", id);        for(i = 0; i < id; i ++){            ans[i] = ori[i];            ans[i + id] = mitoming[ori[i]- 'a'];        }        ans[i + id] = '\0';        printf("%s\n", ans);    }    return 0;}