扫地机器人问题

 先上代码：

#include
#include
#define ll long long
using namespace std;
int a[100001];
int n,m;
bool check(int x){
  int sum = 0;
  for(int i = 1;i <= m;i++){
    if(a[i] - x <= sum)//左边界比要扫的区域小
    {
      if(a[i] <= sum)//机器人在需要打扫的区域内
       sum = a[i] + x - 1;
      else sum = sum + x;
    }
    else return 0;//左边界超出所需打扫区域
  }
  return sum >= n;
}
int main()
{
  cin >> n >> m;
  for(int i = 1;i <= m;i++)
  cin >> a[i];
  sort(a + 1,a + 1 + m);
  int lb = 0, ub = n, mid, ans;
  while(ub >= lb)
  {
    mid=(lb + ub) / 2;
    if(check(mid))
    {
      ub = mid - 1;
      ans = mid;
    }
    else
    lb=mid+1;
  }
  cout << (ans - 1) * 2 << endl;
  return 0;
}