Compress String

/*
aaabb -> a3b2
 */

/*
test case
aaabb

result -> a3
size = 5
i = 0, j = 1, c = 1
j = 2 c = 2
j = 3 c = 3 j - > b

c = 1, i = 3, j = 4
j = 5 c = 2

 */
public class solution{

    public String compressString(String input) {
        if (input == null || input.length() <= 1) {
            return input;
        }

        int size = input.length();
        int i = 0;
        int j = 1;
        int count = 1;
        StringBuilder result = new StringBuilder();

        while (j < size) {
            count = 1;
            while (j < size && input.charAt(i) == input.charAt(j)) {
                j++;
                count++;
            }

            result.append(input.charAt(i));
            result.append(Integer.toString(count));
            i = j;
            j++;
        }
        return result.toString();

    }
}