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;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		boolean[] black = new boolean[n];
		boolean[] black2 = new boolean[n];
		String str = sc.next();
		for (int i = 0; i < n; i++) {
			if (str.charAt(i) == '#') {
				black[i] = true;
				black2[i] = true;
			}
		}
		
		int count = exec(black, black2, 0, 0, 0);
		
		System.out.println(count);
	}
	
	public static int exec(boolean[] array, boolean[] array2, int index, int count1, int count2) {
		if (index >= array.length - 1) {
			return Math.min(count1, count2);
		} else {
			if (!array[++index] && array[index - 1]) {
				++count1;
				array[index] = !array[index];
			}
			if (!array2[index] && array2[index - 1]) {
				++count2;
				array2[index - 1] = !array2[index - 1];
			}
			return exec(array, array2, index, count1, count2);
		}
	}
}