题目链接：uva 12206 - Stammering Aliens

题目大意：给出一个字符串，找出至少出现m次的最长子串。

解题思路：哈希算法，将每一个后缀数组建立一个哈希值。每次二分长度推断，每次推断时将哈希值排序，计数就可以。

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

using namespace std;

typedef unsigned long long ll;
const int maxn = 40005;
const int x = 123;

int N, M, pos, Rank[maxn];
char str[maxn];
ll h[maxn], xp[maxn], Hash[maxn];

void init () {
    scanf("%s", str);
    N = strlen(str);

    h[N] = 0;
    for (int i = N - 1; i >= 0; i--)
        h[i] = h[i+1] * x + str[i] - ‘a‘;
    xp[0] = 1;
    for (int i = 1; i <= N; i++)
        xp[i] = xp[i-1] * x;
}

bool cmp (const int& a, const int& b) {
    return Hash[a] < Hash[b] || (Hash[a] == Hash[b] && a < b);
}

bool judge (int l) {
    int c = 0, n = N - l + 1;
    pos = -1;

    for (int i = 0; i < n; i++) {
        Rank[i] = i;
        Hash[i] = h[i] - h[i+l] * xp[l];
    }

    sort (Rank, Rank + n, cmp);
    for (int i = 0; i < n; i++) {
        if (i == 0 || Hash[Rank[i]] != Hash[Rank[i-1]]) c = 0;
        if (++c >= M)
            pos = max(pos, Rank[i]);
    }
    return pos >= 0;
}

void bsearch () {
    if (!judge(1)) {
        printf("none\n");
        return;
    }

    int l = 1, r = N + 1;
    while (r - l > 1) {
        int mid = (r + l) / 2;
        if (judge(mid))
            l = mid;
        else
            r = mid;
    }
    judge(l);
    printf("%d %d\n", l, pos);
}

int main () {
    while (scanf("%d", &M) == 1 && M) {
        init();
        bsearch();
    }
    return 0;
}