240531 문자열 교환

public static void main(String[] args) throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String input = br.readLine();

	int minSwaps;
	int aCount = 0;

	for (int i = 0; i < input.length(); i++) {
		if (input.charAt(i) == 'a') {
			aCount++;
		}
	}

	int bCount = 0;
	for (int i = 0; i < aCount; i++) {
		if (input.charAt(i) == 'b') {
			bCount++;
		}
	}
	minSwaps = bCount;

	for (int i = 1; i < input.length(); i++) {
		if (input.charAt(i - 1) == 'b') {
			bCount--;
		}
		if (input.charAt((i + aCount - 1) % input.length()) == 'b') {
			bCount++;
		}
		minSwaps = Math.min(minSwaps, bCount);
	}

	System.out.println(minSwaps);
}

출처:https://www.acmicpc.net/problem/1522