Coding

Reverse Words in a String II

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1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

Given a character array 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 a single space.

Your code must solve the problem in-place, i.e. without allocating extra space.

Input: s = ["t","h","e"," ","s","k","y"," ","i","s"," ","b","l","u","e"]

Step 1: Reverse the entire array

Original:
["t","h","e"," ","s","k","y"," ","i","s"," ","b","l","u","e"]
After reversing:
["e","u","l","b"," ","s","i"," ","y","k","s"," ","e","h","t"]

Step 2: Reverse each word individually

Reverse "eulb" → "blue"
Reverse "si" → "is"
Reverse "yks" → "sky"
Reverse "eht" → "the" Final Output: s = ["b","l","u","e"," ","i","s"," ","s","k","y"," ","t","h","e"]

Input: s = ["a"] Output: ["a"] Explanation: Since there is only one character (one word), no changes are needed.

Input : s = ["c","o","d","e"," ","i","s"," ","f","u","n"]

1 <= s.length <= 105
s[i] is an English letter (uppercase or lowercase), digit, or space ' '.
There is at least one word in s.
s does not contain leading or trailing spaces.
All the words in s are guaranteed to be separated by a single space.

cpp

class Solution {
public:
    void reverseWords(vector<char>& s) {
        // Your code goes here
    }
};

java

class Solution {
    public void reverseWords(char[] s) {
        // Your code goes here
    }
}

python

class Solution(object):
    def reverseWords(self, s):
        """
        :type s: List[str]
        :rtype: None Do not return anything, modify s in-place instead.
        """
        # Your code goes here

javascript

/**
 * @param {character[]} s
 * @return {void} Do not return anything, modify s in-place instead.
 */
var reverseWords = function(s) {
    // Your code goes here
};

csharp

public class Solution
{
    public void ReverseWords(char[] s)
    {
        // Your code goes here
    }
}

func reverseWords(s []byte) {
    //your code goes here
}

Stuck? Show a way to structure it+

01Clarify whether words are separated by single spaces.
02Reverse the full character array.
03Scan each word boundary.
04Reverse every word segment.
05Return the transformed array or string.

Reference answer

For the usual in-place version with one space between words, reverse the entire character array first. This puts words in reverse order but reverses each word's letters. Then scan from left to right: when a word starts, advance to its terminating space or end, and reverse that segment. The second reversal restores letters within each word while preserving the new word order. For example, reversing "the sky" yields "yks eht"; reversing its two segments yields "sky the". If the input may contain leading, trailing, or repeated spaces, clarify whether they should be preserved or normalized before selecting the implementation, because that changes the contract. Each character participates in a constant number of swaps, so time is O(n) and extra space is O(1) for a mutable character array. Splitting and joining is simpler but violates a strict in-place requirement.

Then expect these follow-ups

How would you normalize multiple spaces in place?
Tests: complexity analysis
What differs for immutable strings?
Tests: follow-up reasoning

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