151. Reverse Words in a String

MediumLeetCode

Given an input string s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Example 1

Input: s = "the sky is blue"

Output: "blue is sky the"

Example 2

Input: s = " hello world "

Output: "world hello"

Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3

Input: s = "a good example"

Output: "example good a"

Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Constraints

• 1 <= s.length <= 10^4
• s contains English letters (upper-case and lower-case), digits, and spaces ' '
• There is at least one word in s

How to solve the problem

Code

class Solution:
    def reverseWords(self, s: str) -> str:
        return " ".join(s.split()[::-1])

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

Complexity

• Approach 1: Split and Reverse Approach

• Time complexity: O(n)

  • We split the string and reverse the words, which takes O(n) time

• Space complexity: O(n)

  • We store the split words in an array, which takes O(n) space

• Approach 2: Two Pointers Approach

• Time complexity: O(n)

  • We traverse the string once, which takes O(n) time

• Space complexity: O(n)

  • We store the result string, which takes O(n) space