LeetCode 151: Reverse Words in a String — Python Solution

Solve LeetCode 151: Reverse Words in a String in Python with a token normalization approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Split on whitespace to discard extra spaces, reverse the word sequence, and join with single spaces.

Algorithm

1. Split on whitespace to discard extra spaces, reverse the word sequence, and join with single spaces.
2. Maintain this invariant: The output contains exactly the processed words in reverse order with normalized spacing.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def reverseWords(self, s):
        return " ".join(reversed(s.split()))

Why this is correct

The proof follows the maintained state: The output contains exactly the processed words in reverse order with normalized spacing. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n) time and O(n) space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

Leading, trailing, and repeated internal spaces are removed.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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