Problem:
Score : 300 points

Problem Statement

There are N stones arranged in a row. Every stone is painted white or black.
A string S represents the color of the stones. The i-th stone from the left is white if the i-th character of S is ., and the stone is black if the character is #.

Takahashi wants to change the colors of some stones to black or white so that there will be no white stone immediately to the right of a black stone.
Find the minimum number of stones that needs to be recolored.

Constraints

1 \leq N \leq 2\times 10^5

S is a string of length N consisting of . and #.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the minimum number of stones that needs to be recolored.

Sample Input 1

3
#.#

Sample Output 1

1

It is enough to change the color of the first stone to white.

Sample Input 2

5
#.##.

Sample Output 2

2

Sample Input 3

9
.........

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
        var N = sc.ReadInt();
        var S = sc.ReadStr();

        var bRight = new int[N];
        var sum = 0;
        for (int i = N - 1; i >= 0; i--)
        {
            if (S[i] == '.') sum++;
            bRight[i] = sum;
        }
        var wLeft = new int[N];
        sum = 0;
        for (int i = 0; i < N; i++)
        {
            if (S[i] == '#') sum++;
            wLeft[i] = sum;
        }

        var ans = Math.Min(bRight[0], wLeft[N - 1]);
        for (int i = 0; i < N - 1; i++)
        {
            ans = Math.Min(ans, wLeft[i] + bRight[i + 1]); ;
        }
        Console.WriteLine(ans);
    }

    static Scanner sc = new Scanner();
}

class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }
    public string ReadStr()
    {
        return this.Next();
    }
    public char ReadChar()
    {
        return this.Next()[0];
    }
    public int ReadInt()
    {
        return int.Parse(this.Next());
    }
    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }
    public long ReadLong()
    {
        return long.Parse(this.Next());
    }
    public double ReadDouble()
    {
        return double.Parse(this.Next());
    }
    public Tuple<int, int> ReadTup()
    {
        return Tuple.Create(this.ReadInt(), this.ReadInt());
    }
    public Tuple<int, int, int> ReadTup3()
    {
        return Tuple.Create(this.ReadInt(), this.ReadInt(), this.ReadInt());
    }
    public Tuple<int, int, int, int> ReadTup4()
    {
        return Tuple.Create(this.ReadInt(), this.ReadInt(), this.ReadInt(), this.ReadInt());
    }
    public Tuple<long, long> ReadTupLong()
    {
        return Tuple.Create(this.ReadLong(), this.ReadLong());
    }
    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }
    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }
    public double[] ReadDoubleArray(int n)
    {
        var array = new double[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadDouble();
        }
        return array;
    }
    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }
    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
    public Tuple<int, int>[] ReadTupArray(int n)
    {
        var array = new Tuple<int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup();
        }
        return array;
    }
    public Tuple<int, int, int>[] ReadTup3Array(int n)
    {
        var array = new Tuple<int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup3();
        }
        return array;
    }
    public Tuple<int, int, int, int>[] ReadTup4Array(int n)
    {
        var array = new Tuple<int, int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup4();
        }
        return array;
    }
    public Tuple<long, long>[] ReadTupLongArray(int n)
    {
        var array = new Tuple<long, long>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTupLong();
        }
        return array;
    }
}