MediumTwo PointersString

Reverse Words in a String

1 approach, code in every language

Given a string made of words separated by spaces, produce a new string in which the order of the words is reversed. A word is any maximal run of non-space characters.

The result must contain the words in reverse order joined by exactly one space, with no leading or trailing spaces and no repeated spaces between words, even if the input had extra spacing.

Example 1

Input: s = "the sky is blue"

Output: "blue is sky the"

The four words are listed from last to first with single spaces between them.

Example 2

Input: s = " hello world "

Output: "world hello"

Surrounding and repeated interior spaces are collapsed while the two words swap positions.

Constraints

1 ≤ s.length ≤ 10^4
s contains English letters, digits, and spaces.
There is at least one word in s.

Split, Filter, and Reverse

Break the string into words while discarding empty fragments caused by extra spaces, reverse the list of words, then join them back with a single space.

Loading animation…

Intuition

The messy part of this problem is the irregular spacing, so the cleanest mental model is to first recover the pure list of words and forget about the original whitespace entirely. Once we hold the words as separate items, reversing their order and rejoining with one space each guarantees the required formatting. Languages with rich string libraries can do this in a couple of lines, but the same idea works with a manual scan.

Algorithm walkthrough

1Scan the string and collect each maximal block of non-space characters as one word, ignoring the gaps between them.
2Discard any empty pieces so leading, trailing, or doubled spaces do not create blank words.
3Reverse the collected list of words.
4Concatenate the words together, inserting a single space between consecutive words.
5Return the resulting string.

Implementation

class Solution:
    def reverseWords(self, s: str) -> str:
        words = s.split()
        words.reverse()
        return ' '.join(words)

Time

O(n)

Space

O(n)

We scan the input once and store the words in an auxiliary structure of size proportional to the input.

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