Language: C++

using System;
using System.Collections.Generic;
using System.Linq;

namespace Baumkuchen {
	class Baumkuchen {
		static void Main(string[] args) {
			System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
			
			//*
			System.IO.StreamReader sr = new System.IO.StreamReader("input33.txt");
			int N = int.Parse(sr.ReadLine());
			long[] v_list = new long[N + 1];
			v_list[0] = 0;
			for (int n = 1; n < N+1; n++) {
				v_list[n] = v_list[n - 1] + long.Parse(sr.ReadLine());
			}
			sr.Close();
			/*/
			int N = int.Parse(Console.ReadLine());
			long[] v_list = new long[N + 1];
			v_list[0] = 0;
			for (int n = 1; n < N + 1; n++) {
				v_list[n] = v_list[n - 1] + long.Parse(Console.ReadLine());
			}
			//*/
			sw.Start();
			Baumkuchen baumkuchen = new Baumkuchen(v_list);
			Console.WriteLine(baumkuchen.GetAnwser());

			sw.Stop();
			Console.WriteLine(sw.ElapsedMilliseconds);
		}

		readonly long[] v_list;
		readonly int N;
		readonly double tripartition;
		public Baumkuchen(long[] _v_list) {
			N = _v_list.Length - 1;
			v_list = _v_list;
			tripartition = v_list[N] / 3.0;
		}
		public long GetAnwser() {
			long interm_maxV = -1;
			int n0_max = findIndex(0, N, tripartition)[0];
			for (int n0 = 0; n0 <= n0_max; n0++){
				long tmp = searchOne(n0);
				if (tmp > interm_maxV) interm_maxV = tmp;
			}
			return interm_maxV;
		}

		private long searchOne(int n0) {
			int[] n1 = findIndex(n0, N, tripartition + v_list[n0]);
			int[] n2 = findIndex(n1[0], N, 2 * tripartition + v_list[n0]);
			return (from nn1 in n1 from nn2 in n2 select getMinV(n0, nn1, nn2)).Max();
		}

		private long getMinV(int n0, int n1, int n2) {
			if (n1 == -1 || n2 == -1) return long.MaxValue;
			else {
				long[] V0_V1_V2 = new long[] { 
					v_list[n1] - v_list[n0], 
					v_list[n2] - v_list[n1], 
					v_list[n0] - v_list[n2] + v_list[N] };
				return V0_V1_V2.Min();
			}
		}

		private int[] findIndex(int from_n, int to_n, double target) {
			int n_left = from_n;
			int n_right = to_n;
			while (n_right - n_left >1) {
				int n_mid = (n_right + n_left) / 2;
				if (v_list[n_mid] <= target) n_left = n_mid;
				else n_right = n_mid;
			}
			return new int[] { n_left, n_right };
		}
	}
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Linq;
      |       ^~~~~~
a.cc:7:34: error: 'string' has not been declared
    7 |                 static void Main(string[] args) {
      |                                  ^~~~~~
a.cc:7:43: error: expected ',' or '...' before 'args'
    7 |                 static void Main(string[] args) {
      |                                           ^~~~
a.cc:35:17: error: 'readonly' does not name a type
   35 |                 readonly long[] v_list;
      |                 ^~~~~~~~
a.cc:36:17: error: 'readonly' does not name a type
   36 |                 readonly int N;
      |                 ^~~~~~~~
a.cc:37:17: error: 'readonly' does not name a type
   37 |                 readonly double tripartition;
      |                 ^~~~~~~~
a.cc:38:23: error: expected ':' before 'Baumkuchen'
   38 |                 public Baumkuchen(long[] _v_list) {
      |                       ^~~~~~~~~~~
      |                       :
a.cc:38:42: error: expected ',' or '...' before '_v_list'
   38 |                 public Baumkuchen(long[] _v_list) {
      |                                          ^~~~~~~
a.cc:43:23: error: expected ':' before 'long'
   43 |                 public long GetAnwser() {
      |                       ^~~~~
      |                       :
a.cc:53:24: error: expected ':' before 'long'
   53 |                 private long searchOne(int n0) {
      |                        ^~~~~
      |                        :
a.cc:59:24: error: expected ':' before 'long'
   59 |                 private long getMinV(int n0, int n1, int n2) {
      |                        ^~~~~
      |                        :
a.cc:70:24: error: expected ':' before 'int'
   70 |                 private int[] findIndex(int from_n, int to_n, double target) {
      |                        ^~~~
      |                        :
a.cc:70:28: error: expected unqualified-id before '[' token
   70 |                 private int[] findIndex(int from_n, int to_n, double target) {
      |                            ^
a.cc:80:10: error: expected ';' after class definition
   80 |         }
      |          ^
      |          ;
a.cc: In static member function 'static void Baumkuchen::Baumkuchen::Main(int*)':
a.cc:8:25: error: 'System' was not declared in this scope
    8 |                         System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
      |                         ^~~~~~
a.cc:12:33: error: expected primary-expression before 'int'
   12 |                         int N = int.Parse(sr.ReadLine());
      |                                 ^~~
a.cc:13:29: error: structured binding declaration cannot have type 'long int'
   13 |                         long[] v_list = new long[N + 1];
      |                             ^~
a.cc:13:29: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:13:29: error: empty structured binding declaration
a.cc:13:32: error: expected initializer before 'v_list'
   13 |                         long[] v_list = new long[N + 1];
      |                                ^~~~~~
a.cc:14:25: error: 'v_list' was not declared in this scope
   14 |                         v_list[0] = 0;
      |                         ^~~~~~
a.cc:16:61: error: expected primary-expression before 'long'
   16 |                                 v_list[n] = v_list[n - 1] + long.Parse(sr.ReadLine());
      |                                                             ^~~~
a.cc:18:25: error: 'sr' was not declared in this scope
   18 |                         sr.Close();
      |                         ^~
a.cc:27:25: error: 'sw' was not declared in this scope
   27 |                         sw.Start();
      |                         ^~
a.cc:29:25: error: 'Console' was not declared in this scope
   29 |                         Console.WriteLine(baumkuchen.GetAnwser());
      |                         ^~~~~~~
a.cc: In constructor 'Baumkuchen::Baumkuchen::Baumkuchen(long int*)':
a.cc:39:25: error: 'N' was not declared in this scope
   39 |                         N = _v_list.Length - 1;
      |                         ^
a.cc:39:29: error: '_v_list' was not declared in this scope
   39 |                         N = _v_list.Length - 1;
      |                             ^~~~~~~
a.cc:40:25: error: 'v_list' was not declared in this scope
   40 |                         v_list = _v_list;
      |                         ^~~~~~
a.cc:41:25: error: 'tripartition' was not declared in this scope
   41 |                         tripartition = v_list[N] / 3.0;
      |                         ^~~~~~~~~~~~
a.cc: In member function 'long int Baumkuchen::Baumkuchen::GetAnwser()':
a.cc:45:51: error: 'N' was not declared in this scope
   45 |                         int n0_max = findIndex(0, N, tripartition)[0];
      |                                                   ^
a.cc:45:54: error: 'tripartition' was not declared in this scope
   45 |                         int n0_max = findIndex(0, N, tripartition)[0];
      |                                                      ^~~~~~~~~~~~
a.cc:45:38: error: 'findIndex' was not declared in this scope
   45 |                         int n0_max = findIndex(0, N, tripartition)[0];
      |                                      ^~~~~~~~~
a.cc: In member function 'long int Baumkuchen::Baumkuchen::searchOne(int)':
a.cc:54:28: error: structured binding declaration cannot have type 'int'
   54 |                         int[] n1 = findIndex(n0, N, tripartition + v_list[n0]);
      |                            ^~
a.cc:54:28: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:54:28: error: empty structured binding declaration
a.cc:54:31: error: expected initializer before 'n1'
   54 |                         int[] n1 = findIndex(n0, N, tripartition + v_list[n0]);
      |                               ^~
a.cc:55:28: error: structured binding declaration cannot have type 'int'
   55 |                         int[] n2 = findIndex(n1[0], N, 2 * tripartition + v_list[n0]);
      |                            ^~
a.cc:55:28: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:55:28: error: empty structured binding declaration
a.cc:55:31: error: expected initializer before 'n2'
   55 |                         int[] n2 = findIndex(n1[0], N, 2 * tripartition + v_list[n0]);
      |                               ^~
a.cc:56:33: error: 'from' was not declared in this scope
   56 |                         return (from nn1 in n1 from nn2 in n2 select getMinV(n0, nn1, nn2)).Max();
      |                                 ^~~~
a.cc:56:38: error: expected ')' before 'nn1'
   56 |                         return (from nn1 in n1 from nn2 in n2 select getMinV(n0, nn1, nn2)).Max();
      |                                      ^~~
a.cc:56:32: note: to match this '('
   56 |                         return (from nn1 in n1 from nn2 in n2 select getMinV(n0, nn1, nn2)).Max();
      |                                ^
a.cc: In member function 'long int Baumkuchen::Baumkuchen::getMinV(int, int, int)':
a.cc:60:58: error: expected primary-expression before 'long'
   60 |                         if (n1 == -1 || n2 == -1) return long.MaxValue;
      |                                                          ^~~~
a.cc:60:58: error: expected ';' before 'long'
a.cc:60:62: error: expected unqualified-id before '.' token
   60 |                         if (n1 == -1 || n2 == -1) return long.MaxValue;
      |                                                              ^
a.cc:61:25: error: 'else' without a previous 'if'
   61 |                         else {
      |                         ^~~~
a.cc:62:37: error: structured binding declaration cannot have type 'long int'
   62 |                                 long[] V0_V1_V2 = new long[] {
      |                                     ^~
a.cc:62:37: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:62:37: error: empty structured binding declaration
a.cc:62:40: error: expected initializer before 'V0_V1_V2'
   62 |                                 long[] V0_V1_V2 = new long[] {
      |                                        ^~~~~~~~
a.cc:66:40: error: 'V0_V1_V2' was not declared in this scope
   66 |                                 return V0_V1_V2.Min();
      |                                        ^~~~~~~~