【动态规划】[luoguP1868]饥饿的奶牛

题目

一开始又没用读懂题
DP题 dp[i]表示牛走到i这个位置能吃到的最大草数 最后ans找最大值
关键找最右的端点 然后从0一直找过去(为什么从0找我也不知道 一开始从1找wa了一个大点 从0找就ac了)

代码如下

#include<iostream>#include<cstdio>#include<cctype>#include<algorithm>    using namespace std;    #define in = read();    typedef long long ll;    typedef unsigned int ui;    const ll size = 5000000 + 10000;        struct data{    ll x , y;}a[size];            int n;            ll rightpoint , j = 1 , ans;            ll f[size];inline ll read(){        ll num = 0 , f = 1;    char ch = getchar();        while(!isdigit(ch)){                if(ch == '-')   f = -1;                ch = getchar();        }        while(isdigit(ch)){                num = num*10 + ch - '0';                ch = getchar();        }        return num*f;}inline bool cmp(data a , data b){        return a.x != b.x ? a.x < b.x : a.y < b.y;}int main(){        n in;        for(register int i=1;i<=n;i++){                a[i].x in;  a[i].y in;                rightpoint = max(rightpoint , a[i].y);        }        sort(a + 1 , a + n + 1 , cmp);        for(register int i=0;i<=rightpoint;i++){                f[i] = max(f[i] , f[i - 1]);                while(a[j].x == i && j <= n){                        f[a[j].y] = max(f[a[j].y] , f[a[j].x - 1] + a[j].y - a[j].x + 1);                        j ++;                }                ans = max(ans , f[i]);        }        printf("%lld" , ans);}//COYG