151. Reverse Words in a String
Description
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 <= 104
s
contains English letters (upper-case and lower-case), digits, and spaces' '
.- There is at least one word in
s
.
Follow-up: If the string data type is mutable in your language, can you solve it in-place with O(1)
extra space?
Solutions
Solution 1: Use Language Built-in Functions
We split the string into a list of strings by spaces, then reverse the list, and finally join the list into a string separated by spaces.
Time complexity $O(n)$, space complexity $O(n)$, where $n$ is the length of the string.
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Solution 2: Two Pointers
We can use two pointers $i$ and $j$, each time we find a word, add it to the result list, then reverse the result list, and finally join the list into a string.
Time complexity $O(n)$, space complexity $O(n)$, where $n$ is the length of the string.
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