### PROBLEM LINK:

**Author:** nmalviya1929

**Tester:** manjunath1996

**Editorialist:** nmalviya1929

### DIFFICULTY:

SIMPLE

### PREREQUISITES:

None

### PROBLEM:

Given a string s consisting of **a** and **b**. Find number of subsequence “ab” in s.

### QUICK EXPLANATION:

This problem can be simplified as for each b, find the total number of a that precedes it. Answer is the summation of all the values.

### EXPLANATION:

Question can be simplified as for a particular b_{i} ,we have to find all the subsequence "a_{j}b_{i}" where 0<=j< i .This value is equal to number of a's preceding b.Answer is summation of all this values.

To calculate number of a's that precede each b, we will keep a variable count that will keep track of number of a's encountered so far while traversing string from left to right. While traversing string from left to right , whenever an 'a' is encountered, we will increment count variable and wheneber 'b' is encountered we will increment answer by count.

### AUTHOR’S AND TESTER’S SOLUTIONS:

Author’s solution can be found here.

Tester’s solution can be found here.