Program.cs

using static System.Console;
using static System.Linq.Enumerable;
using CompetitiveCSharp;

namespace CSharpTest
{
    static class Program
    {
        static void WriteTree(SegTree tree)
        {
            for(var i = 0; i < tree.Length; ++i)
            {
                for(var j = 0; j <= tree.Length; ++j)
                {
                    Write($"{tree.Get(i, j)} ");
                }
                WriteLine();
            }
        }
        
        static void Main(string[] args)
        {
            var test = new int[8];
            foreach(var i in Range(0, 8))
            {
                test[i] = i + 1;
            }
            var tree = new SegTree(test, (a, b) => a + b);
            WriteTree(tree);
            WriteLine();

            tree[1] = 9;
            tree[3] = 10;
            tree[5] = 11;
            tree[7] = 12;
            WriteTree(tree);
        }
    }
}

SegTree.cs

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Linq.Enumerable;

namespace CompetitiveCSharp
{
    static class SegTreeHelp
    {
        const int check_count = 1000000;
        public static int GetUnit(this Func<int, int, int> op)
        {
            var random = new Random();
            var kouho = new int[] { 0, 1, int.MinValue, int.MaxValue };
            foreach (var u in kouho)
            {
                var f = true;
                foreach (var c in Range(0, check_count))
                {
                    var v = random.Next();
                    if (v != op(v, u) || v != op(u, v))
                    {
                        f = false;
                        break;
                    }
                }
                if (f)
                {
                    return u;
                }
            }
            throw new ArgumentException("与えられた関数は単位元を持ちません");
        }
    }

    public class SegTree
    {
        int[] vec;
        Func<int, int, int> op;
        public int Length { get; }
        int unit;
        
        static int GetSize(int s)
        {
            if ((s & -s) == s)
            {
                return s;
            }
            while ((s & -s) != s)
            {
                s -= s & -s;
            }
            return s << 1;
        }
        public SegTree(int size, Func<int, int, int> op)
        {
            this.vec = new int[2 * GetSize(size)];
            this.op = op;
            this.unit = this.op.GetUnit();
            this.Length = this.vec.Length / 2;
            foreach(var i in Range(0, 2 * this.Length))
            {
                this.vec[i] = this.unit;
            }
        }
        public SegTree(IEnumerable<int> init, Func<int, int, int> op) : this(init.Count(), op)
        {
            var i = this.Length;
            foreach (var v in init)
            {
                this.vec[i++] = v;
            }
            foreach(var index in Range(1, this.Length - 1).Reverse())
            {
                this.vec[index] = op(this.vec[2 * index], this.vec[2 * index + 1]);
            }
        }

        public int Get(int a, int b)
        {
            var L = this.unit;
            var R = this.unit;
            for (a += this.Length, b += this.Length; a < b; a >>= 1, b >>= 1)
            {
                if ((a & 1) != 0)
                {
                    L = this.op(L, this.vec[a++]);
                }
                if ((b & 1) != 0)
                {
                    R = this.op(this.vec[--b], R);
                }
            }
            return this.op(L, R);
        }

        public int this[int index]
        {
            get
            {
                return this.vec[this.Length + index];
            }

            set
            {
                this.vec[this.Length + index] = value;
                for(var s = (this.Length + index) / 2; s > 0; s /= 2)
                {
                    this.vec[s] = this.op(this.vec[2 * s], this.vec[2 * s + 1]);
                }
            }
        }
    }
}