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 Rust (1.15.1) (judge verdict: Accepted)

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    let mut iter = buf.split_whitespace();
    let n: usize = iter.next().unwrap().parse().unwrap();

    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    let s = buf.trim();

    let mut nr_wb = Vec::<(usize, usize)>::with_capacity(n);

    for c in s.chars() {
        let (w, b) = *nr_wb.last().unwrap_or(&(0usize, 0usize));
        if c == '.' {
            nr_wb.push((w+1, b));
        }
        else {
            nr_wb.push((w, b+1));
        }
    }

    let mut ans_wb: usize = n;
    let (all_w, _) = *nr_wb.last().unwrap();
    for i in 0..(n-1) {
        let (cur_w, cur_b) = nr_wb[i];
        let rem_w = all_w - cur_w;
        ans_wb = std::cmp::min(ans_wb, cur_b + rem_w);
    }

    let ans_w: usize = nr_wb.last().unwrap().1;
    let ans_b: usize = nr_wb.last().unwrap().0;

    println!("{}", std::cmp::min(std::cmp::min(ans_w, ans_b), ans_wb));
}