Number of subsequences of the form a^i b^j c^k

Given a string, count number of subsequences of the form aibjck, i.e., it consists of i ’a’ characters, followed by j ’b’ characters, followed by k ’c’ characters where i >= 1, j >=1 and k >= 1.

Note: Two subsequences are considered different if the set of array indexes picked for the 2 subsequences are different.

Expected Time Complexity: O(n)

Examples:

Input  : abbc
Output : 3
Subsequences are abc, abc and abbc

Input  : abcabc
Output : 7
Subsequences are abc, abc, abbc, aabc
abcc, abc and abc

class Solution {
    public int countSubsequences(String s) {
        // Initialize counts of different subsequences 
        // caused by different combination of 'a' 
        int aCount = 0; 
        // Initialize counts of different subsequences 
        // caused by different combination of 'a' and 
        // different combination of 'b' 
        int bCount = 0; 
        // Initialize counts of different subsequences 
        // caused by different combination of 'a', 'b' 
        // and 'c'. 
        int cCount = 0; 
        // Traverse all characters of given string 
        for (int i = 0; i < s.length(); i++) { 
            /* If current character is 'a', then 
               there are following possibilities : 
                 a) Current character begins a new 
                    subsequence. 
                 b) Current character is part of aCount 
                    subsequences. 
                 c) Current character is not part of 
                    aCount subsequences. */
            if (s.charAt(i) == 'a') 
                aCount = (1 + 2 * aCount); 
            /* If current character is 'b', then 
               there are following possibilities : 
                 a) Current character begins a new 
                    subsequence of b's with aCount 
                    subsequences. 
                 b) Current character is part of bCount 
                    subsequences. 
                 c) Current character is not part of 
                    bCount subsequences. */
            else if (s.charAt(i) == 'b') 
                bCount = (aCount + 2 * bCount); 
  
            /* If current character is 'c', then 
               there are following possibilities : 
                 a) Current character begins a new 
                    subsequence of c's with bCount 
                    subsequences. 
                 b) Current character is part of cCount 
                    subsequences. 
                 c) Current character is not part of 
                    cCount subsequences. */
            else if (s.charAt(i) == 'c') 
                cCount = (bCount + 2 * cCount); 
        } 
        return cCount; 
    } 
}