luogu_P5019 铺设道路

维护单调不上升栈，贪心

#include<iostream>
#include<cstdio>

#define ri register int
#define u int

namespace opt {

    inline u in() {
        u x(0),f(1);
        char s(getchar());
        while(s<'0'||s>'9') {
            if(s=='-') f=-1;
            s=getchar();
        }
        while(s>='0'&&s<='9') {
            x=(x<<1)+(x<<3)+s-'0';
            s=getchar();
        }
        return x*f;
    }

}

using opt::in;

#include<algorithm>
#include<cstring>

#define NN 100005

namespace mainstay {

    u q[NN],r,ans;

    inline void solve() {
        u N(in());
        for(ri i(1); i<=N; ++i) {
            u _a(in());
            if(_a<=q[r]) q[++r]=_a;
            else {
                ans+=_a-q[r];
                while(r-1>=0&&q[r-1]>_a) --r;
                q[++r]=_a;
            }
        }
        std::cout<<ans;
    }

}

int main() {

    //freopen("x.txt","r",stdin);
    std::ios::sync_with_stdio(false);
    mainstay::solve();

}