额，虽然过了，但还是有点问题

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<cmath>
#include<stdlib.h>
#define INF 0x3f3f3f3f
#define MAXN 1000002
using namespace std;
int n,m;
int sum=0;
int a[MAXN];
int l,r;

bool check(int s)
{
    int tot=0,num=1;
    for (int i=1;i<=n;i++)
    {
        tot+=a[i];
        if (a[i]>s)
            return 0;
        if (tot>s)
        {
            tot=a[i];
            num++;
            if (num>m)
                return 0;
        }
    }
    return num<=m;
}

int main()
{
    scanf("%d%d",&n,&m);
    l=INF,r;
    for (int i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
        sum+=a[i];
        l=min(l,a[i]);
    }
    r=sum;
    while (l+1<r)
    {
        int mid=(l+r)>>1;
        if (check(mid))
            r=mid;
        else
            l=mid;
    }
    for (int i=l;i<=r;i++)
        if (check(i))
            l=i;
    printf("%d\n",l);
    return 0;
}