UVA 12206 - Stammering Aliens

题目链接

题意：给定一个序列，求出出现次数大于m，长度最长的子串的最大下标

思路：后缀数组。搞出height数组后，利用二分去查找就可以

这题之前还写过hash的写法也能过，只是写后缀数组的时候，犯了一个傻逼错误，把none输出成node还一直找不到。。。这是刷题来第二次碰到这样的逗比错误了，还是得注意。

。

代码：

#include <cstdio>

#include <cstring>

#include <algorithm>

using namespace std;

const int MAXLEN = 40005;

struct Suffix {

    int s[MAXLEN];

    int sa[MAXLEN], t[MAXLEN], t2[MAXLEN], c[MAXLEN], n;

    int rank[MAXLEN], height[MAXLEN];

    void build_sa(int m) {

	n++;

	int i, *x = t, *y = t2;

	for (i = 0; i < m; i++) c[i] = 0;

	for (i = 0; i < n; i++) c[x[i] = s[i]]++;

	for (i = 1; i < m; i++) c[i] += c[i - 1];

	for (i = n - 1; i >= 0; i--) sa[--c[x[i]]] = i;

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

	    int p = 0;

	    for (i = n - k; i < n; i++) y[p++] = i;

	    for (i = 0; i < n; i++) if (sa[i] >= k) y[p++] = sa[i] - k;

	    for (i = 0; i < m; i++) c[i] = 0;

	    for (i = 0; i < n; i++) c[x[y[i]]]++;

	    for (i = 0; i < m; i++) c[i] += c[i - 1];

	    for (i = n - 1; i >= 0; i--) sa[--c[x[y[i]]]] = y[i];

	    swap(x, y);

	    p = 1; x[sa[0]] = 0;

	    for (i = 1; i < n; i++)

		x[sa[i]] = (y[sa[i - 1]] == y[sa[i]] && y[sa[i - 1] + k] == y[sa[i] + k]) ?

p - 1 : p++;

	    if (p >= n) break;

	    m = p;

	}

	n--;

    }

    void getHeight() {

	int i, j, k = 0;

	for (i = 1; i <= n; i++) rank[sa[i]] = i;

	for (i = 0; i < n; i++) {

	    if (k) k--;

	    int j = sa[rank[i] - 1];

	    while (s[i + k] == s[j + k]) k++;

	    height[rank[i]] = k;

	}

    }

} gao;

const int N = 40005;

int m;

char str[N];

int judge(int x) {

    int ans = -1;

    for (int i = 1; i <= gao.n; i++) {

	if (gao.n - gao.sa[i] < x) continue;

	int Max = gao.sa[i], cnt = 1;

	while (gao.height[i + 1] >= x && i < gao.n) {

	    Max = max(Max, gao.sa[i + 1]);

	    cnt++;

	    i++;

	}

	if (cnt >= m)

	    ans = max(ans, Max);

    }

    return ans;

}

void solve() {

    if (judge(1) == -1) {

	printf("none\n");

	return;

    }

    int l = 1, r = gao.n - m + 2;

    while (l < r) {

	int mid = (l + r) / 2;

	if (judge(mid) != -1) l = mid + 1;

	else r = mid;

    }

    l--;

    printf("%d %d\n", l, judge(l));

}

int main() {

    while (~scanf("%d", &m) && m) {

	scanf("%s", str);

	int len = strlen(str);

	for (int i = 0; i < len; i++)

	    gao.s[i] = str[i] - 'a' + 1;

	gao.s[len] = 0;

	gao.n = len;

	gao.build_sa(27);

	gao.getHeight();

	solve();

    }

    return 0;

}