Count Sub-sequences in a String

MediumPython

Problem

Write a program to count sub-sequences in a string.

For this task, count all subsequences containing exactly two characters. A subsequence is formed by choosing two distinct character positions while preserving their order. Characters with the same value at different positions are counted separately.

Input: A string s.

Output: Return the number of length-two subsequences as an integer.

For a string of length n, every pair of positions forms one valid subsequence.

Constraints

0 <= len(s) <= 10^5
s contains any standard Python string characters
Subsequences are counted by index selection, not by distinct resulting text

Function Signature

def count_subsequences(s):

CodePython 3

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Run your code to see test output here.