The First Encounter

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 to the index of its first occurrence.

Every character has a story - but only if you remember where it started.

Example 1

Input:

s = "hello"

Output:

{"h":0,"e":1,"l":2,"o":4}

Example 2

Input:

s = "abc"

Output:

{"a":0,"b":1,"c":2}

Worked solution and explanation

Why this problem exists in real interviews

This tests first-occurrence tracking with a dict, a pattern used in deduplication and data quality tools. It probes whether a candidate correctly skips updates for already-seen keys.

Break down the requirements

Step 1: Iterate through the string with index tracking

Process each character with its position.

Step 2: Record only the first occurrence

If the character is not yet in the dict, add it with the current index. If it is already present, skip it.

The solution

First-occurrence index mapping

def first_occurrence(s: str) -> dict:
    result = {}
    for i in range(len(s)):
        ch = s[i]
        if ch not in result:
            result[ch] = i
    return result

Common follow-up questions

How would you return the last occurrence instead? (Tests removing the guard so every occurrence overwrites the previous.)
What if you needed both first and last occurrence? (Tests maintaining two dicts or a dict of tuples.)
How would you handle this for a streaming input? (Tests that first-occurrence tracking works naturally in streaming since you only need to check if seen before.)