美团笔试第四道，能否帮忙看看哪里有问题

用的DFS，但是只能过60%

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int num = in.nextInt();
        int[] nums = new int[num];

        String in_ = in.next();
        char[] c = in_.toCharArray();
        for (int i=0;i<num;i++) {
            nums[i] = c[i]-96;
        }

        List<Integer> x_list = new ArrayList<>();

        for (int i=1;i<=num;i++) {
            if (num%i==0) {
                x_list.add(i);
            }
        }

        int _max = Integer.MAX_VALUE;
        for (int x: x_list) {
            int y = num/x;
            int res = f(x, y, nums);
            _max = Math.min(_max, res);
        }
        System.out.println(_max);
    }

    public static int f(int x, int y, int[] nums) {
        int[][] s = new int[x][y];

        int n = 0;
        for (int i=0;i<x;i++) {
            for (int j=0;j<y;j++) {
                s[i][j] = nums[n];
                n++;
            }
        }

        int res = 0;
        boolean[][] used = new boolean[x][y];

        int col = s.length;
        int line = s[0].length;
        for (int i=0;i<col;i++) {
            for (int j = 0; j < line; j++) {
                if (!used[i][j]) {
                    dfs(s, used, i, j);
                    res++;
                }
            }
        }
        return res;
    }

    public static void dfs(int[][] nums, boolean[][] used, int i, int j) {
        if (used[i][j]) return;

        int col = nums.length;
        int row = nums[0].length;
        used[i][j] = true;

        if (i + 1 < col && !used[i + 1][j] && nums[i][j] == nums[i + 1][j]) {
            dfs(nums, used, i+1, j);
        }
        if (i - 1 >= 0 && !used[i - 1][j] && nums[i][j] == nums[i - 1][j]) {
            dfs(nums, used, i-1, j);
        }
        if (j + 1 < row && !used[i][j+1] && nums[i][j] == nums[i][j+1]) {
            dfs(nums, used, i, j+1);
        }
        if (j - 1 >= 0 && !used[i][j-1] && nums[i][j] == nums[i][j-1]) {
            dfs(nums, used, i, j=1);
        }
    }
}