【BZOJ3100】排列【杂项】

【题目链接】

同【BZOJ1318】

注意会卡空间...

/* Footprints In The Blood Soaked Snow */
#include <cstdio>
#include <algorithm>

using namespace std;

typedef unsigned int uint;
typedef long long LL;

const int maxn = 1000005, inf = 0x3f3f3f3f;

int n, last[maxn], next[maxn], ans;
uint sum[maxn];
bool vis[maxn];

inline int iread() {
	int f = 1, x = 0; char ch = getchar();
	for(; ch < '0' || ch > '9'; ch = getchar()) f = ch == '-' ? -1 : 1;
	for(; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
	return f * x;
}

inline void calc(int x) {
	int len = 0, r = inf;
	for(int i = x + 1; i <= n && sum[i] - sum[i - 1] != 1; i++) {
		int now = sum[i] - sum[i - 1];
		if(!vis[now]) vis[now] = 1;
		else {
			r = i;
			break;
		}
	}
	for(int i = x; i >= 1; i--) {
		int now = sum[i] - sum[i - 1];
		if(now == 1 && i != x) break;
		len = max(len, now);
		r = min(r, next[i]);
		if(i + len - 1 < r && i + len - 1 <= n)
			if(sum[i + len - 1] - sum[i - 1] == (LL)len * (len + 1) >> 1)
				ans = max(ans, len);
	}
	for(int i = x + 1; i <= n && sum[i] - sum[i - 1] != 1; i++)
		vis[sum[i] - sum[i - 1]] = 0;
}

inline void solve() {
	for(int i = 1; i <= n; i++) last[i] = inf;
	for(int i = n; i >= 1; i--) {
		int now = sum[i] - sum[i - 1];
		next[i] = last[now];
		last[now] = i;
	}
	for(int i = 1; i <= n; i++) if(sum[i] - sum[i - 1] == 1)
		calc(i);
}

int main() {
	n = iread();
	for(int i = 1; i <= n; i++) sum[i] = sum[i - 1] + iread();
	solve();

	for(int i = n; i >= 1; i--) sum[i] = sum[i] - sum[i - 1];
	reverse(sum + 1, sum + 1 + n);
	for(int i = 1; i <= n; i++) sum[i] += sum[i - 1];
	solve();

	printf("%d\n", ans);
	return 0;
}