【代填】土地购买「Usaco2008 Mar」（贪心+斜率优化dp）

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 50010;
template <typename T> void read(T &x) {
	T f = 1;
	char ch = getchar();
	for (; '0' > ch || ch > '9'; ch = getchar()) if (ch == '-') f = -1;
	for (x = 0; '0' <= ch && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
	x *= f;
}
int n;
struct node{
	ll w, l;
	friend bool operator < (node x, node y) {
		if (x.w != y.w) return x.w < y.w;
		return x.l < y.l;
	}
}a[N], r[N];
int top;
ll dp[N];
int q[N], head, tail;
long double Y(int i) {
	return dp[i];
}
long double X(int i) {
	return r[i + 1].l;
}
long double slope(int i, int j) {
	if (X(i) == X(j)) {
		return Y(j) >= Y(i) ? 1e18 : -1e18;
	}
	return (Y(j) - Y(i)) / (X(j) - X(i));
}
int main() {
	read(n);
	for (int i = 1; i <= n; i++) read(a[i].w), read(a[i].l);
	sort(a + 1, a + 1 + n);
	for (int i = 1; i <= n; r[++top] = a[i], i++) while (top && r[top].l < a[i].l) top--;
	q[head = tail = 1] = 0;
	for (int i = 1; i <= top; i++) {
		while (head < tail && slope(q[head + 1], q[head]) >= -1.0 * r[i].w) head++;
		dp[i] = r[i].w * r[q[head] + 1].l + dp[q[head]];
		while (head < tail && slope(q[tail], q[tail - 1]) <= slope(i, q[tail])) tail--;
		q[++tail] = i;
	}
	printf("%lld", dp[top]);
	return 0;
}