Submission #3895457

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;

void chmax(int64_t& a, int64_t b){
    a = max(a, b);
}

const int M = 200001;
int L, N;
int64_t X[M], Y[M], Xs[M], Ys[M];

int main(){
    cin >> L >> N;
    for(int i=1; i<=N; i++){
        cin >> X[i];
        Y[N+1-i] = L - X[i];
    }
    for(int i=1; i<=N; i++){
        Xs[i] = Xs[i-1] + X[i];
        Ys[i] = Ys[i-1] + Y[i];
    }

    const int dx[2] = {1, 0};
    const int dy[2] = {0, 1};
    int64_t ans = 0;
    for(int d=0; d<=N; d++){
        int xt = d, yt = N-d;
        for(int k1=0; k1<2; k1++){
            for(int k2=0; k2<2; k2++){
                int cx = min(xt, yt + (- dy[k1] - dy[k2] + dx[k1] + dx[k2]) / 2);
                int cy = min(yt, xt + (- dx[k1] - dx[k2] + dy[k1] + dy[k2]) / 2);
                if(cx < 0 || cy < 0) continue;

                int64_t result = 0;
                result += (Xs[xt] - Xs[xt-cx]) * 2;
                result += (Ys[yt] - Ys[yt-cy]) * 2;
                result -= (k2 ? Y[yt] : X[xt]);
                chmax(ans, result);
            }
        }
    }
    cout << ans << endl;
    return 0;
}

Submission Info

Judge Result