Return lexicographically maximum possible proper string

Question

Obtain a proper string that is lexicographically maximum. In one operation, you can swap a character It is given that a string is called a proper string if it has equal counts of 'a','b','c','d'. You can swap a character of the string with any of 'a','b','c','d'.

    import java.util.*;
class Main
{
static int MAX = 26;
static String smallestStr(char []str, int n)
{
    int i, j = 0;
    int []chk = new int[MAX];
    for (i = 0; i < MAX; i++)
        chk[i] = -1;
    for (i = 0; i < n; i++)
    {
        if (chk[str[i] - 'a'] == -1)
            chk[str[i] - 'a'] = i;
    }
    for (i = 0; i < n; i++)
    {
        boolean flag = false;
        for (j = 0; j < str[i] - 'a'; j++)
        {
            if (chk[j] > chk[str[i] - 'a'])
            {
                flag = true;
                break;
            }
        }
        if (flag)
            break;
    }
    if (i < n)
    {
        char ch1 = str[i];
        char ch2 = (char) (j + 'a');
        for (i = 0; i < n; i++)
        {
            if (str[i] == ch1)
                str[i] = ch2;

            else if (str[i] == ch2)
                str[i] = ch1;
        }
    }

    return String.valueOf(str);
}

public static void main(String[] args)
{
    String str = "aacacbbc";
    int n = str.length();
    System.out.println(smallestStr(
                    str.toCharArray(), n));
}
}

The output is supposed to be dadacbbc but my output is aababccb