spoj 1811 Longest Common Substring（后缀自动机）

题目链接：spojm 1811 Longest Common Substring

代码

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
typedef long long ll;
const int maxn = 250005;
const int SIGMA_SIZE = 26;

struct SAM {
    int sz, last;
    int g[maxn<<1][SIGMA_SIZE], pre[maxn<<1], step[maxn<<1];
    int tot;

    void newNode(int s) {
        step[++sz] = s;
        pre[sz] = 0;
        memset(g[sz], 0, sizeof(g[sz]));
    }

    int idx(char ch) { return ch -'a'; }

    void init() {
        tot = 0;
        sz = 0, last = 1;
        newNode(0);
    }

    int insert(char ch) {
        newNode(step[last] + 1);
        int v = idx(ch), p = last, np = sz;

        while (p && !g[p][v]) {
            g[p][v] = np;
            p = pre[p];
        }

        if (p) {
            int q = g[p][v];
            if (step[q] == step[p] + 1)
                pre[np] = q;
            else {
                newNode(step[p] + 1);
                int nq = sz;
                for (int j = 0; j < SIGMA_SIZE; j++) g[nq][j] = g[q][j];

                pre[nq] = pre[q];
                pre[np] = pre[q] = nq;

                while (p && g[p][v] == q) {
                    g[p][v] = nq;
                    p = pre[p];
                }
            }
        } else
            pre[np] = 1;
        tot += step[np] - step[pre[np]];
        last = np;
        return tot;
    }

    int solve(char* str) {
        int n = strlen(str), p = 1, ans = 0, lcs = 0;

        for (int i = 0; i < n; i++) {
            int v = idx(str[i]);
            if (g[p][v]) p = g[p][v], lcs++;
            else {
                while (p && g[p][v] == 0) p = pre[p];
                if (p) lcs = step[p] + 1, p = g[p][v];
                else lcs = 0, p = 1;
            }
            ans = max(ans, lcs);
        }
        return ans;
    }
}SA;

char a[maxn], b[maxn];

int main () {
    while (scanf("%s%s", a, b) == 2) {
        int n = strlen(a);
        SA.init();
        for (int i = 0; i < n; i++) SA.insert(a[i]);
        printf("%d\n", SA.solve(b));
    }
    return 0;
}