题目： P7404 [JOI 2021 Final] とてもたのしい家庭菜園 4

#include <iostream>
#include <algorithm>
using namespace std;
const int N = 200010;
typedef long long LL;
LL S[N],a[N];
LL n;
LL ans = 0;
int main()
{
    cin >> n;
    for(int i = 1;i <= n;i ++) 
    {
        cin >> S[i];
        a[i] = S[i] - S[i - 1];
    }
    LL l = 1,r = n;
    while(l < r)
    {
        while(a[l] > 0) l ++;
        while(a[r] < 0) r --;
        if(l > r) 
        {
            break;
        }
        else
        {
            LL x = min(1 - a[l],a[r] + 1);
            ans += x;
            a[l] += x;
            a[r] -= x;
        }
    }
    cout << ans << endl;
    return 0;
}