基础赛 T4 悬关求调

#include <bits/stdc++.h>
using namespace std;
#define int long long
int b[1000005], c[1000005];
bool ff[1000005];
signed main() {
    int n, a, d; cin >> n >> a >> d;
    for(int i = 1;i < n;i++)
        cin >> b[i], b[i] = (b[i] - 2) / (2 * d), c[i] = b[i];
    // for(int i = 1;i < n;i++) cout << b[i] << " \n"[i == n-1];
    for(int i = 1;i < n;i++) if(!b[i]) ff[i] = 1;
    int ans3 = 0;
    for(int i = a;i < n;) {
        int can = b[i];
        ans3 += can; 
        if(i < (n-1) && b[i+1] >= 1) i++;
        else if(i < (n-2) && b[i+2] >= 1) i++, i++;
        else break;
    } int ans4 = 0; for(int i = a;i >= 1;) {
        int can = b[i];
        ans4 += can; 
        if(i >= 2 && b[i-1] >= 1) i--;
        else if(i >= 3 && b[i-2] >= 1) i--, i--;
        else break;
    }
    int p = a, lst = a, ans = 0;
    for(;p < n;) {
        ans += max(b[p] - 1, 0ll); b[p] = min(b[p], 1ll);
        if(p < (n-1) && b[p+1] > 1) lst++, p++;
        else if(p < (n-2) && b[p+2] > 1) lst += 2, p += 2;
        else {
            if(ff[p]) {
                if(p < (n-1) && b[p+1] == 1) {
                    if(b[p-1] > 0) ans++;
                } break;
            }
            if(p < (n-1) && b[p+1] == 1) {
                b[p+1] = 0; ans++;
                if(p < (n-2) && b[p+2] == 1) {
                    p++; continue;
                } else break;
            } else if(p < (n-2) && b[p+2] == 1 && lst == p) {
                b[p+2] = 0; ans++; break;
            } else break;
        }
    } for(;p >= 1;) {
        ans += b[p]; b[p] = 0;
        if(p >= 2 && b[p-1] > 0) p--;
        else if(p >= 3 && b[p-2] > 0) p -= 2;
        else break;
    } int ans2 = 0;
    p = a, lst = a, ans2 = 0;
    for(;p >= 1;) {
        ans2 += max(c[p] - 1, 0ll); c[p] = min(c[p], 1ll);
        if(p >= 2 && c[p-1] > 1) lst--, p--;
        else if(p >= 3 && c[p-2] > 1) lst -= 2, p -= 2;
        else {
            if(ff[p]) {
                if(p >= 2 && b[p-1] == 1) {
                    if(b[p+1] > 0) ans2++;
                } break;
            }
            if(p >= 2 && c[p-1] == 1) {
                c[p-1] = 0; ans2++;
                if(p > 2 && c[p-2] > 0) { p--; continue; } 
                else break;
            } else if(p >= 3 && c[p-2] == 1 && lst == p) {
                c[p-2] = 0; ans2++; break;
            } else break;
        }
    } // cout << ans2 << " " << p << endl;
    // for(int i = 1;i < n;i++) cout << c[i] << " \n"[i == n-1]; 
    for(;p < n;) {
        ans2 += c[p]; c[p] = 0;
        if(p < (n-1) && c[p+1] > 0) p++;
        else if(p < (n-2) && c[p+2] > 0) p += 2;
        else break;
    } cout << max({ans, ans2, ans3, ans4}) << endl;
    return 0;
}