kinderGartenAdventure - hackerRank - score 30 (maximum score 30) - segment tree - binary indexed tree

kindergartenAdventure_studycodeB.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
class Solution {
    static void Main(String[] args) 
    {
        int n = int.Parse(Console.ReadLine());
        var a = Console.ReadLine().Split().Select(int.Parse).ToArray();
        
        var tree = new SegmentTree(n);
        for (int i=0; i<n; i++)
        {
            int v = a[i];
            if (v>=a.Length) continue;
            int start = (i + 1) % a.Length;
            int end = (i + a.Length - v) % a.Length;
            if (start<=end)
                tree.Modify(start, end-start+1, 1);
            else
            {
                tree.Modify(start, a.Length-start, 1);
                tree.Modify(0, end+1, 1);
            }
        }
        
        int bestIndex = 1;
        int best = 0;
        for (int i=1; i<=a.Length; i++)
        {
            var v = tree.Query(i-1);
            //Console.WriteLine($"{i}->{v}");
            if (v>best)
            {
                best = v;
                bestIndex = i;
            }
        }

        Console.WriteLine(bestIndex);
    }
    
    public class SegmentTree
    {
        private readonly int[] _tree;

        public SegmentTree(int size)
        {
            _tree = new int[size*2];
        }

        public void Modify(int start, int count, int value)
        {
            int size = _tree.Length/2;
            int left = start + size;
            int right = start + count + size; // open border
            for (; left < right; left >>= 1, right >>= 1)
            {
                if (left%2 == 1) _tree[left++] += value;
                if (right%2 == 1) _tree[--right] += value;
            }
        }

        public int Query(int index)
        {
            int res = 0;
            int i =  index + _tree.Length/2;
            for (; i > 0; i >>= 1)
                res += _tree[i];
            return res;
        }

    }
}