poj 3017 Cut the Sequence

      #include<iostream>

#include<cstdio>

#include<cstring>

#include<string>

#include<map>

#include<vector>

#include<stack>

#include<set>

#include<map>

#include<queue>

#include<deque>

#include<algorithm>

#include<cmath>

#define LL long long

//#pragma comment(linker, "/STACK:1024000000,1024000000")

using namespace std;

const int INF=0x3f3f3f3f;

const int N=400005;

LL a[N];

int n;

LL m,ans[N];

int dq[N],il,ir;

int main()

{

    //freopen("data.in","r",stdin);

    scanf("%d %I64d",&n,&m);

    for(int i=1;i<=n;++i)

    scanf("%I64d",&a[i]);

    for(int i=1;i<=n;++i)

    if(a[i]>m)

    {printf("-1\n");return 0;}

    il=0;ir=-1;

    LL sum=0;

    a[0]=0;

    ans[0]=0;

    int l=1;

    for(int i=1;i<=n;++i)

    {

        while(il<=ir&&a[dq[ir]]<=a[i])

        --ir;

        dq[++ir]=i;

        sum+=a[i];

        while(sum>m)

        {

            sum-=a[l];

            ++l;

        }

        while(dq[il]<l)

        {++il;}

        if(l==1)

        {ans[i]=a[dq[il]];continue;}

        ans[i]=ans[l-1]+a[dq[il]];

        for(int j=il;j<ir;++j)

        {

            int k=dq[j];

            ans[i]=min(ans[i],ans[k]+a[dq[j+1]]);

        }

    }

    printf("%I64d\n",ans[n]);

    return 0;

}