The Letter Frequency Map

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3
Asked in
Nintendo

Problem

Given a string, return a dict mapping each distinct character (including spaces and punctuation) to its count.

Count every character in the string and report the results.

Example 1

Input:

s = "hello"

Output:

{"h":1,"e":1,"l":2,"o":1}

Example 2

Input:

s = "aab"

Output:

{"a":2,"b":1}

Worked solution and explanation

Why this problem exists in real interviews

This tests the most basic frequency counting pattern. It probes whether a candidate can build a frequency dict from scratch without relying on collections.Counter.

Break down the requirements

Step 1: Initialize an empty dict

Map each character to its count.

Step 2: Iterate and count

For each character, increment its count in the dict.

The solution

Manual frequency dict construction
def char_frequency(s: str) -> dict:
    freq = {}
    for ch in s:
        if ch in freq:
            freq[ch] += 1
        else:
            freq[ch] = 1
    return freq

Common follow-up questions