LeetCode 151. Reverse Words in a String



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

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.

Example 4:

Input: s = "  Bob    Loves  Alice   "
Output: "Alice Loves Bob"

Example 5:

Input: s = "Alice does not even like bob"
Output: "bob like even not does Alice"


  • 1 <= s.length <= 104
  • s contains English letters (upper-case and lower-case), digits, and spaces ' '.
  • There is at least one word in s.

Follow up:

  • Could you solve it in-place with O(1) extra space?


Reversed and build a new string.

Python Solution

class Solution:
    def reverseWords(self, s: str) -> str:
        result = ""
        words = s.split()
        for i, word in enumerate(words[::-1]):
            if i != 0:
                result += " "
            result += word
        return result
  • Time Complexity: O(N).
  • Space Complexity: O(1).

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