51NOD1243排船問題
題意
$\mathrm{一}\mathrm{個}\mathrm{碼}\mathrm{頭}\mathrm{中}\mathrm{有}N\mathrm{艘}\mathrm{船}$

#include<stdio.h>
#include<iostream>
#include<algorithm>
using namespace std;
#define dbg(x) cout<<#x<<" = "<<x<<endl
const int maxn = 5e4+10;
int a[maxn];
int n,x,m;
bool check(int mid)
{
    int head=0,tail=0;
    for(int i=1;i<=n;i++)
    {
        head=tail;
        if(head+x-mid<=a[i])
        {
            if(head+x+mid<=a[i])
            {
                head=a[i]-x-mid;
                tail=head+2*x;
            }
            else
            {
                tail+=2*x;
            }
        }
        else
        {
            return 0;
        }
    }
    return tail<=m;
}
int main()
{
    scanf("%d%d%d",&n,&x,&m);
    for(int i=1;i<=n;i++) scanf("%d",&a[i]);
    int l=0,r=m-1;
    while(l<=r)
    {
        int mid=(l+r)/2;
        if(check(mid)) r=mid-1;
        else l=mid+1;
    }
    if(l==m||2*n*x>m) printf("-1\n");
    else printf("%d\n",l);
    return 0;
}