URAL 1297 Palindrome (最长回文子串）

1297. Palindrome

Input

Output

Sample

ThesampletextthatcouldbereadedthesameinbothordersArozaupalanalapuazorA

ArozaupalanalapuazorA

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <climits>
#include <string>

using namespace std;

#define MP make_pair
#define PB push_back
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef pair<int, int> PII;
typedef pair<double, double> PDD;
const int INF = INT_MAX / 3;
const double eps = 1e-8;
const LL LINF = 1e17;
const double DINF = 1e60;
const int maxn = 5005;

//以下是倍增法求后缀数组
int wa[maxn], wb[maxn], wv[maxn], ws[maxn];
int cmp(int *r, int a, int b, int l) { return r[a] == r[b] && r[a + l] == r[b + l]; }
void da(int *r, int *sa, int n, int m) {
int i, j, p, *x = wa, *y = wb, *t;
for(i = 0; i < m; i++) ws[i] = 0;
for(i = 0; i < n; i++) ws[x[i] = r[i]]++;
for(i = 1; i < m; i++) ws[i] += ws[i - 1];
for(i = n - 1; i >= 0; i--) sa[--ws[x[i]]] = i;
for(j = 1, p = 1; p < n; j <<= 1, m = p) {
for(p = 0, i = n - j; i < n; i++) y[p++] = i;
for(i = 0; i < n; i++) if(sa[i] >= j) y[p++] = sa[i] - j;
for(i = 0; i < n; i++) wv[i] = x[y[i]];
for(i = 0; i < m; i++) ws[i] = 0;
for(i = 0; i < n; i++) ws[wv[i]]++;
for(i = 0; i < m; i++) ws[i] += ws[i - 1];
for(i = n - 1; i >= 0; i--) sa[--ws[wv[i]]] = y[i];
for(t = x, x = y, y = t, p = 1, x[sa[0]] = 0, i = 1; i < n; i++)
x[sa[i]] = cmp(y, sa[i - 1], sa[i], j) ? p - 1 : p++;
}
}

//以下是求解height 数组
int height[maxn], Rank[maxn];
void calheight(int *r, int *sa, int n) {
int i, j, k = 0;
for(i = 1; i <= n; i++) Rank[sa[i]] = i;
for(i = 0; i < n; height[Rank[i++]] = k)
for(k ? k-- : 0, j = sa[Rank[i] - 1];
r[i + k] == r[j + k]; k++) ;
}

char buf[maxn];
int str[maxn], sa[maxn], len, n;
int minv[maxn][20];

void init_RMQ() {
for(int i = 0; i <= len; i++) minv[i][0] = height[i];
for(int j = 1; (1 << j) <= len + 1; j++) {
for(int i = 0; i + (1 << j) - 1 <= len; i++) {
minv[i][j] = min(minv[i][j - 1], minv[i + (1 << (j - 1))][j - 1]);
}
}
}

int query(int ql, int qr) {
if(ql > qr) swap(ql, qr);
ql++;
int k = 0;
while((1 << (k + 1)) <= qr - ql + 1) k++;
return min(minv[ql][k], minv[qr - (1 << k) + 1][k]);
}

int main() {
while(scanf("%s", buf) != EOF) {
n = len = strlen(buf);
for(int i=0;i<len;i++) str[i] = buf[i];
str[len] = '*';
for(int i=n-1;i>=0;i--) str[++len] = buf[i];
str[++len] = 0;
da(str, sa, len + 1, 200);
calheight(str, sa, len);
init_RMQ();
int anspos=0, anslen=1;
for(int i=0;i<n;i++)
{
int a = query(Rank[i],Rank[len-i-1]);
int b = a*2-1;
if(b>anslen)
{
anslen = b;
anspos = i-(a-1);
}
a = query(Rank[i],Rank[len-i]);
b = a*2;
if(b>anslen)
{
anslen = b;
anspos = i-a;
}
}
for(int i=anspos;i<anspos+anslen;i++) putchar(buf[i]);
puts("");
}
return 0;
}