UVA 12206 - Stammering Aliens(Hash+LCP)

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UVA 12206 - Stammering Aliens

题目链接

题意：给定一个字符串，找出重复出现超过m次的字串的最大开始下标

思路：hash大法，需要点人品，然后二分答案，每次利用hash值去找出最大下标即可

代码：

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;typedef unsigned long long ull;const ull x = 123;const int N = 40005;int m, n;char str[N];ull H[N], Hp[N];void gethash() {    H[n] = 0;    for (int i = n - 1; i >= 0; i--)H[i] = H[i + 1] * x + str[i] - 'a';}struct Suf {    int hash, id;} suf[N];bool cmp(Suf a, Suf b) {    if (a.hash == b.hash) return a.id < b.id;    return a.hash < b.hash;}int find(int L) {    for (int i = 0; i < n - L + 1; i++) {suf[i].hash = H[i] - H[i + L] * Hp[L];suf[i].id = i;    }    sort(suf, suf + n - L + 1, cmp);    int ans = -1;    for (int i = 0; i < n - L + 1; i++) {int j, len = 0;for (j = i; suf[i].hash == suf[j].hash && j < n - L + 1; j++)    len++;if (len >= m) {    ans = max(ans, suf[j - 1].id);}i = j - 1;    }    return ans;}void solve() {    if (find(1) == -1) printf("none\n");    else {int l = 1, r = n - m + 2;while (l < r) {    int mid = (l + r) / 2;    if (find(mid) == -1) r = mid;    else l = mid + 1;}printf("%d %d\n", l - 1, find(l - 1));    }}int main() {    Hp[0] = 1;    for (int i = 1; i < N; i++)Hp[i] = Hp[i - 1] * x;    while (~scanf("%d", &m) && m) {scanf("%s", str);n = strlen(str);gethash();solve();    }    return 0;}

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