Changes in a string

You are given a string $S$ of $N$ characters comprising of $A^{\prime }s$ and $B^{\prime }s$. You can choose any index $i$ and change $S_i$ to either $A$ or $B$.

Find the minimum number of changes that you must make to string $S$ such that the resulting string is of the following format:

 $\underset{x\text{ number of }{A}^{\prime }s}{\underbrace{AAAA........}}\underset{n-x\text{ number of }{B}^{\prime }s}{\underbrace{BBBB........}}$       

where $0\le x\le n$

In other words, your task is to determine the minimum number of changes such that string $S$ has $x$ number of $A^{\prime }s$ in the beginning, followed by the remaining $(n-x)$ number of $B^{\prime }s$.

Example:

Input: n=4, str="BABA"
Output: 2

Approach

Java

public class ChangesInstring {

    public static void main(String args[]) throws Exception {
        int n = 4;
        String str = "BABA";
        int ans = check(str, str.length());
        System.out.println(ans);
    }

    static int check(String s, int n) {
        int count = 0, a = 0, b = 0;
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) == 'A') {
                a++;
                if (b != 0 && a > b) {
                    count += b;
                    a += b;
                    b = 0;
                }
            } else {
                if (b == 0) {
                    a = 0;
                    b++;
                } else
                    b++;
            }
        }
        if (a != 0 && b != 0)
            count = (a > b) ? count + b : count + a;
        return count;
    }
}