How to add number of character Occurrences inside a string

My task is to find number of occurrences of a string character and add that number inside the string

public static void main(String[] args) {

    char[] arr = "hello".toCharArray();
    
    arr[2] = '1';
    arr[3] = '2';
    
    System.out.println(arr);
}

Output should be: he12o

I know we cant reuse this approach.

>Solution :

what is the output of "helololol"?

output for helololol, ch=’l’ , then the output should be he1o2o3o4; if ch=’o’ then output should be hel1l2l3l

If according to this rule, Can be achieved with a loop:

public static void main(String[] args) {
    char flag = 'l';
    String str = "hellollololollol";
    int num = 1;
    for(int i = 0, len = str.length(); i < len; i++) {
        if (str.charAt(i) == flag) {
            str = str.substring(0, i) + num++ + str.substring(i + 1);
        }
    }
    System.out.println(str);
}

Note that if the number of specified characters exceeds 9, it will look weird, If the number of characters exceeds 9, special processing is required：

public static void main(String[] args) {
    char flag = 'l';
    String str = "hellollololollollol";
    int num = 1;
    for(int i = 0, len = str.length(); i < len; i++) {
        if (str.charAt(i) == flag) {
            str = str.substring(0, i) + num++ + str.substring(i + 1);
            if (num > 10) {
                len++;
            }
        }
        System.out.println(str);
    }
}

The same problem, if the number of characters exceeds 100, 1000, 10000, special processing is required, because the length of the number added to the string is one bit longer than the original character, how to deal with it flexibly, you need to think about it yourself！