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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.Scanner;
import java.util.ArrayList;

public final class Main{
    public static void main(String[] args){
        Scanner stdIn = new Scanner(System.in);

        int n = stdIn.nextInt();
        String s = stdIn.next();

        char[] ca = s.toCharArray();

        ArrayList<Integer> grope = new ArrayList<>();
        boolean firstIsWhite = (ca[0] == '.') ? true : false;
        int counter = 1;
        char lastlit = ca[0];
        for(int i=1; i<n; i++){
            if(lastlit == ca[i]){
                counter++;
            }else{
                grope.add(counter);
                counter = 1;
            }
            lastlit = ca[i];
        }
        grope.add(counter);

        int ans = 0;

        if((firstIsWhite && grope.size()%2 == 0) || (!firstIsWhite && grope.size()%2 ==1)){
            for(int i=grope.size()-1; i>=0; i = i-2){
                if(i == grope.size()-1){

                }else{
                    if(grope.get(i)<grope.get(i+1)){
                        ans++;
                        if(i != 0){
                            grope.set(i-1, grope.get(i-1) + grope.get(i) + grope.get(i+1));
                        }
                    }else{
                        ans++;
                    }
                }
            }
        }else{
            for(int i=grope.size()-1; i>=0; i = i-2){
                if(i == grope.size()-1){
                    if(i ==0){

                    }else{
                        if(grope.get(i-1)<grope.get(i)){
                            ans++;
                            if(i != 0 && i!=1){
                                grope.set(i-2, grope.get(i-2) + grope.get(i-1) + grope.get(i));
                            }
                        }else{
                            ans++;
                        }
                    }
                }else if(i == 0){
                    //何もしない
                }else{
                    if(grope.get(i-1)<grope.get(i)){
                        ans++;
                        if(i != 0 && i!=1){
                            grope.set(i-2, grope.get(i-2) + grope.get(i-1) + grope.get(i));
                        }
                    }else{
                        ans++;
                    }
                }
            }
        }

        System.out.println(ans);
    }
}