MiddleDebuggingRare

Why is this string-building loop slow on large input?

This works, but an interviewer flags its complexity on large words.

Python

def join_words(words):
    s = ""
    for w in words:
        s += w + " "
    return s.strip()

Find and fix the performance bug.

Strings are immutable, so each s += ... builds a brand-new string — O(n²) for n words, slow on large inputs. It is functionally correct, but the complexity is the bug. Fix: return ' '.join(words), which is O(n) and idiomatic.

Believing += on a string mutates a buffer in place
Blaming the trailing space or strip instead of repeated allocation
Micro-optimizing the concatenation instead of using join

Why does string immutability force a new allocation on each +=?
When is io.StringIO or a list-plus-join preferable for building text?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

Back to questions