AT2273 [ARC066C] Addition and Subtraction Hard

性质：

1. 括号都用在减号后。
2. 减号后加括号除了这个减号和下一个减号之间的数减去，后面所有数都可加上绝对值，显然这个括号最优是右括号放在最后。

故枚举加括号的位置，前缀和优化即可。

时间复杂度 $\mathcal O(n)$。

#include<bits/stdc++.h>

using namespace std;

namespace Fread {
	const int SIZE = 1 << 23;
	char buf[SIZE], *S, *T;
	inline char getchar() {
		if (S == T) {
			T = (S = buf) + fread(buf, 1, SIZE, stdin);
			if (S == T)
				return '\n';
		}
		return *S++;
	}
}

namespace Fwrite {
	const int SIZE = 1 << 23;
	char buf[SIZE], *S = buf, *T = buf + SIZE;
	inline void flush() {
		fwrite(buf, 1, S - buf, stdout);
		S = buf;
	}
	inline void putchar(char c) {
		*S++ = c;
		if (S == T)
			flush();
	}
	struct NTR {
		~NTR() {
			flush();
		}
	} ztr;
}

#ifdef ONLINE_JUDGE
	#define getchar Fread::getchar
	#define putchar Fwrite::putchar
#endif

#define int long long

inline int read() {
	int x = 0, f = 1;
	char c = getchar();
	while (c < '0' || c > '9') {
		if (c == '-')
			f = -1;
		c = getchar();
	}
	while (c >= '0' && c <= '9')
		x = x * 10 + c - '0', c = getchar();
	return x * f;
}

inline void write(int x) {
	if (x < 0) {
		putchar('-');
		x = -x;
	}
	if (x > 9)
		write(x / 10);
	putchar(x % 10 + '0');
}

const int _ = 1e5 + 10;

int n, a, k, b[_], s[_], ss[_];

signed main() {
	n = read();
	b[1] = 0;
	for (int i = 1; i <= n; ++i) {
		if (i != 1) {
			char c = getchar();
			while (c != '+' && c != '-') c = getchar();
			if (c == '-') b[i] = 1, k++;
		}
		a = read();
		if (b[i] == 1) a = -a;
		s[i] = s[i - 1] + a;
		ss[i] = ss[i - 1] + abs(a);
	}
	int lst = n + 1;
	for (int i = 1; i <= n; ++i)
		if (b[i] == 1) {
			lst = i;
			break;
		}
	if (lst == n + 1 || k == 1)
		return printf("%lld", s[n]), 0;
	int ans = s[n];
	for (int i = lst + 1; i <= n; ++i)
		if (b[i] == 1)
			ans = max(ans, s[lst - 1] - (ss[i - 1] - ss[lst - 1]) + ss[n] - ss[i - 1]), lst = i;
	printf("%lld", ans);
	return 0;
}