题目链接：uva 11107 - Life Forms

题目大意：给定n个字符串，求一个最长的字符串，为n/2个字符串的子串。

解题思路：后缀数组，处理除后缀数组后，二分长度，每次遍历height数组，当长度不足时就分段，如果存在一段中包含n/2个起点，则为可行长度。

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

using namespace std;
const int MAXLEN = 200005;

struct Suffix_Arr {
    int n, s[MAXLEN];
    int tmp_one[MAXLEN], tmp_two[MAXLEN], c[MAXLEN];
    int SA[MAXLEN], rank[MAXLEN], height[MAXLEN];

    void init();
    void build_arr(int m);
    void get_height();
}suf;

int N, maxlen, id[MAXLEN];

void init () {
    maxlen = 0;
    suf.init();
    char word[1005];

    for (int i = 0; i < N; i++) {
        scanf("%s", word);

        if (N == 1) {
            printf("%s\n", word);
            return;
        }
        int len = strlen(word);
        maxlen = max(maxlen, len);

        for (int j = 0; j < len; j++) {
            id[suf.n] = i;
            suf.s[suf.n++] = word[j];
        }
        suf.s[suf.n++] = ‘z‘ + i + 1;
    }
    suf.build_arr(‘z‘ + N + 1);
    suf.get_height();
}

bool judge (int l, int flag) {
    set<int> vis;

    vis.insert(id[suf.SA[0]]);
    for (int i = 1; i < suf.n; i++) {
        while (i < suf.n && suf.height[i] >= l)
            vis.insert(id[suf.SA[i++]]);

        if (vis.size() * 2 > N) {
            if (flag)
                return true;

            for (int j = 0; j < l; j++)
                printf("%c", suf.s[suf.SA[i-1] + j]);
            printf("\n");
        }
        vis.clear();
        vis.insert(id[suf.SA[i]]);
    }
    return false;
}

void solve () {

    if (N == 1)
        return;

    if (!judge(1, 1)) {
        printf("?\n");
        return;
    }

    int l = 1, r = maxlen + 1;
    while (l < r) {
        int mid = (l + r) / 2;
        if (judge(mid, 1))
            l = mid + 1;
        else
            r = mid;
    }
    judge(l - 1, 0);
}

int main () {
    int cas = 0;
    while (scanf("%d", &N) == 1 && N) {
        if (cas++)
            printf("\n");

        init();
        solve();
    }
    return 0;
}

void Suffix_Arr::init () {
    n = 0;
}

void Suffix_Arr::get_height () {
    for (int i = 0; i < n; i++) rank[SA[i]] = i;

    int k = 0;
    for (int i = 0; i < n; i++) {
        if (k) k--;

        if (rank[i] == 0)
            continue;

        int j = SA[rank[i]-1];
        while (s[i+k] == s[j+k]) k++;
        height[rank[i]] = k;
    }
}

void Suffix_Arr::build_arr (int m) {

    int *x = tmp_one, *y = tmp_two;

    for (int i = 0; i < m; i++) c[i] = 0;
    for (int i = 0; i < n; i++) c[x[i] = s[i]]++;
    for (int i = 1; i < m; i++) c[i] += c[i-1];
    for (int i = n-1; i >= 0; i--) SA[--c[x[i]]] = i;

    for (int k = 1; k <= n; k <<= 1) {

        int mv = 0;
        for (int i = n - k; i < n; i++) y[mv++] = i;
        for (int i = 0; i < n; i++) if (SA[i] >= k) {
            y[mv++] = SA[i] - k;
        }

        for (int i = 0; i < m; i++) c[i] = 0;
        for (int i = 0; i < n; i++) c[x[y[i]]]++;
        for (int i = 1; i < m; i++) c[i] += c[i-1];
        for (int i = n-1; i >= 0; i--) SA[--c[x[y[i]]]] = y[i];

        swap(x, y);
        mv = 1; x[SA[0]] = 0;
        for (int i = 1; i < n; i++)
            x[SA[i]] = (y[SA[i-1]] == y[SA[i]] && y[SA[i-1] + k] == y[SA[i] + k] ? mv - 1 : mv++);

        if (mv >= n)
            break;
        m = mv;
    }
}

uva 11107 - Life Forms(后缀数组)