字母交换 - 通过代码

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            String S = sc.next();
            int n = sc.nextInt();
            System.out.println(getMaxLength(S, n));
        }
    }

    private static int getMaxLength(String s, int n) {
        ArrayList<ArrayList<Integer>> index = new ArrayList<>();
        for (int i = 0; i < 26; ++i) {
            index.add(new ArrayList<>());
        }

        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            index.get(ch - 'a').add(i);
        }

        int ans = 0;
        for (int i = 0; i < 26; i++) {
            for (int j = 0; j < index.get(i).size(); j++) {
                for (int k = j + 1; k <= index.get(i).size(); ++k) {
                    if (judge(index.get(i), j, k, n)) {
                        ans = Math.max(ans, k - j);
                    }
                }
            }
        }
        return ans;
    }

    private static boolean judge(ArrayList<Integer> index, int from, int end, int n) {
        int mid = (from + end - 1) / 2;
        int left = index.get(mid) - (end - from - 1) / 2;
        int dis = 0;
        for (int i = 0; i < end - from; ++i) {
            int target = left + i;
            int origin = index.get(i + from);
            dis += Math.abs(target - origin);
        }
        return dis <= n;
    }
}