【斜率优化dp】bzoj 1597 土地购买

最近学习斜率优化！然而其实并不明白斜率是如何操作的……还好有小天使带我❤~而且斜率优化好像是有模板的样子，很多题都可以套用模板。
基本的dp方程：
dp[i]=min(dp[j]+land.x[i]*land.y[j+1]);

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;struct Node {    double x, y;}land[50005], temp[50005];long long dp[50005];int queue[50005], n, head = 0, tail = 1, num = 0;bool cmp(const Node &a, const Node &b) {    return a.x > b.x;}double getx(int k, int j) {    return dp[k] - dp[j];}double gety(int k, int j) {    return land[j + 1].x - land[k + 1].x;}int main() {     scanf("%d", &n);    for(int i = 1; i <= n; i++)        scanf("%lf%lf", &temp[i].x, &temp[i].y);    sort(temp + 1, temp + n + 1, cmp);    land[1].y = -1.0;    for(int i = 1; i <= n; i++)        if(temp[i].y > land[num].y) {            num++;            land[num].x = temp[i].x;            land[num].y = temp[i].y;        }    for(int i = 1; i <= num; i++) {        while(head + 1 < tail && getx(queue[head + 1], queue[head]) <= land[i].y * gety(queue[head + 1], queue[head]))            head++;        dp[i] = dp[queue[head]] + land[i].y * land[queue[head] + 1].x;        while(head + 1 < tail && getx(i, queue[tail - 1])*gety(queue[tail - 1], queue[tail - 2]) <= getx(queue[tail - 1], queue[tail - 2]) * gety(i, queue[tail - 1]))            tail--;        queue[tail++] = i;    }    printf("%lld\n", dp[num]);    return 0;}