BZOJ 3916 Baltic 2014 friends Hash

BZOJ 3916 Baltic 2014 friends Hash
Theme

Returns a string that is connected by two identical strings and followed by a letter. Evaluate the original string.

Ideas

Which of the following is the name of the letter added to the enumeration? Based on the hash value, we will discuss the problem in detail. Pay attention to heavy judgment.

CODE

#define _CRT_SECURE_NO_WARNINGS#include 
  
   #include 
   
    #include 
    
     #include #define MAX 2000010#define BASE 23333using namespace std;int length, mid;char s[MAX];unsigned long long _hash[MAX];unsigned long long power[MAX];int cnt;char ans[MAX], now[MAX];unsigned long long h_ans, h_now;inline unsigned long long GetHash(int l, int r){    return _hash[r] - _hash[l - 1] * power[r - l + 1];}inline bool Judge(int dismiss){    static unsigned long long front, tail;    if(dismiss < mid) {        front = GetHash(1, dismiss - 1) * power[mid - dismiss] + GetHash(dismiss + 1, mid);        tail = GetHash(mid + 1, length);    }    else if(dismiss == mid) {        front = GetHash(1, mid - 1);        tail = GetHash(mid + 1, length);    }    else {        front = GetHash(1, mid - 1);        tail = GetHash(mid, dismiss - 1) * power[length - dismiss] + GetHash(dismiss + 1, length);    }    if(front == tail) {        if(h_ans == front)  return false;        h_ans = front;        int top = 0;        if(dismiss <= mid)            for(int i = mid + 1; i <= length; ++i)                ans[++top] = s[i];        else            for(int i = 1; i < mid; ++i)                ans[i] = s[i];        return true;    }    return false;}int main(){    cin >> length;    if(!(length&1)) {        puts("NOT POSSIBLE");        return 0;    }    mid = (length >> 1) + 1;    scanf("%s", s + 1);    power[0] = 1;    for(int i = 1; i <= length; ++i)        power[i] = power[i - 1] * BASE;    for(int i = 1; i <= length; ++i)        _hash[i] = _hash[i - 1] * BASE + s[i];    for(int i = 1; i <= length; ++i) {        cnt += Judge(i);        if(cnt > 1) break;    }    if(!cnt)        puts("NOT POSSIBLE");    if(cnt == 1)        puts(ans + 1);    if(cnt > 1)        puts("NOT UNIQUE");    return 0;}