URAL 1297 Palindrome

  1 #include <iostream>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <cmath>
  5 #include <algorithm>
  6 #include <climits>
  7 #include <vector>
  8 #include <queue>
  9 #include <cstdlib>
 10 #include <string>
 11 #include <set>
 12 #include <stack>
 13 #define LL long long
 14 #define pii pair<int,int>
 15 #define INF 0x3f3f3f3f
 16 using namespace std;
 17 const int maxn = 10000;
 18 int rk[maxn],lcp[maxn],sa[maxn],wb[maxn],wd[maxn],wv[maxn];
 19 int st[maxn][25];
 20 bool cmp(int *r,int i,int j,int k){
 21     return r[i] == r[j] && r[i+k] == r[j+k];
 22 }
 23 void da(int *r,int *sa,int n,int m) {
 24     int i,k,p,*x = rk,*y = wb;
 25     for(i = 0; i < m; ++i) wd[i] = 0;
 26     for(i = 0; i < n; ++i) wd[x[i] = r[i]]++;
 27     for(i = 1; i < m; ++i) wd[i] += wd[i-1];
 28     for(i = n-1; i >= 0; --i) sa[--wd[x[i]]] = i;
 29 
 30     for(p = k = 1; p < n; k <<= 1,m = p) {
 31         for(p = 0, i = n - k; i < n; ++i) y[p++] = i;
 32         for(i = 0; i < n; ++i) if(sa[i] >= k) y[p++] = sa[i] - k;
 33 
 34         for(i = 0; i < m; ++i) wd[i] = 0;
 35         for(i = 0; i < n; ++i) wv[i] = x[y[i]];
 36 
 37         for(i = 0; i < n; ++i) wd[wv[i]]++;
 38         for(i = 1; i < m; ++i) wd[i] += wd[i-1];
 39         for(i = n-1; i >= 0; --i) sa[--wd[wv[i]]] = y[i];
 40 
 41         swap(x,y);
 42         x[sa[0]] = 0;
 43         for(p = i = 1; i < n; ++i)
 44             x[sa[i]] = cmp(y,sa[i-1],sa[i],k)?p-1:p++;
 45     }
 46 
 47 }
 48 void calcp(int *r,int n){
 49     for(int i = 1; i <= n; ++i) rk[sa[i]] = i;
 50     int h = 0;
 51     for(int i = 0; i < n; ++i){
 52         if(h > 0) h--;
 53         for(int j = sa[rk[i]-1];i+h < n && j + h < n; ++h)
 54             if(r[i+h] != r[j+h]) break;
 55         lcp[rk[i]-1] = h;
 56     }
 57 }
 58 void init(int n){
 59     memset(st,0,sizeof(st));
 60     for(int i = 1; i < n; ++i) st[i][0] = lcp[i];
 61     for(int i = 1; 1<<i < n; ++i){
 62         for(int j = 1; j + (1<<i) <= n; j++){
 63             st[j][i] = min(st[j][i-1],st[j+(1<<(i-1))][i-1]);
 64         }
 65     }
 66 }
 67 int query(int s,int t){
 68     s = rk[s];
 69     t = rk[t];
 70     if(s > t) swap(s,t);
 71     if(t > s) t--;
 72     int r = log2(t - s + 1);
 73     return min(st[s][r],st[t-(1<<r)+1][r]);
 74 }
 75 char str[maxn],tmp[maxn];
 76 int r[maxn];
 77 int main() {
 78     while(gets(str)){
 79         int len = strlen(str);
 80         strcpy(tmp,str);
 81         str[len] = ‘#‘;
 82         reverse(tmp,tmp+len);
 83         strcpy(str+len+1,tmp);
 84         for(int i = 0; str[i]; ++i)
 85             r[i] = str[i];
 86         r[len+len+2] = 0;
 87         da(r,sa,len+len+2,128);
 88         calcp(r,len+len+1);
 89         init(len+len+1);
 90         int ans = 0,index = 0;
 91         for(int i = 0; i <= len; ++i){
 92             int t = query(i,2*len-i)*2 - 1;
 93             if(t > ans){
 94                 ans = t;
 95                 index = i;
 96             }
 97             if(i){
 98                 t = query(i,2*len-i+1)*2;
 99                 if(t > ans){
100                     ans = t;
101                     index = i;
102                 }
103             }
104         }
105         if(ans&1){
106             for(int i = index-(ans>>1),t = index + (ans>>1); i <= t; ++i)
107                 putchar(str[i]);
108             putchar(‘\n‘);
109 
110         }else {
111             for(int i = index - (ans>>1),t = index+(ans>>1);i < t; ++i)
112                 putchar(str[i]);
113             putchar(‘\n‘);
114         }
115     }
116     return 0;
117 }