Question 6 of 75 - LeetCode Weekly Challenge
Day 47 of 100 Days LeetCode Challenge

LeetCode Challenge #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.



  • 1 <= s.length <= 104
  • s contains English letters (upper-case and lower-case), digits, and spaces ' '.
  • There is at least one word in s.
C++ Video Solution
Video Solution
C++ Solution
					class Solution {
string reverseWords(string s) {

        int i=s.length()-1;
        string ans="";
        string helper="";

        for(int j=i; j>=0; j--)
            if(s[j]==' ')
                    ans+=" ";

        else if(ans.size()!=0)ans.pop_back();

        return ans;

Java Solution
					class Solution {
    public String reverseWords(String s) {
      String[] arr = s.split("\\s+"); // split on the basis of spaces 
      StringBuilder sb = new StringBuilder("");

      for(int i=arr.length-1;i>=0;i--){
          sb.append(arr[i] + " ");

      // trim function --> remove trailing and leading spaces 

      String ans = sb.toString();
      ans = ans.trim();
      return ans ;
Code Explanation

In this program, we have to reverse the order of words while keeping the word’s direction the same. Only the words will be interchanged.

For this purpose, we will:

1. We initialize an integer variable ‘i’ to the length of the input string minus one (the index of the last character in the string) and we also initialize two empty strings: ‘ans’ and ‘helper’.

2. If we encounter space anywhere in between then check

  • if Helper’s size is greater than 0 ( helper.size&gt;0 ) or not because if helper contains any word that can be our First Space. If yes, it will reverse the order of characters in ‘helper’ using [ ‘reverse( helper .begin(), helper.end()) ]
    And we will add a helper into the ‘ans’ to add a space to separate words and resets the ‘helper’ string NULL string again ( because we need helper just to one word )
  • If the current character is not a space , it directs to ‘helper’. And this is done to collect characters of the current word in reverse order.

3. After the loop, it checks if ‘helper’ is not empty . If ‘helper’ is not empty , it means the last word has been collected , and it will be added to the ‘ans’ .

• The code reverses the the order of characters in ‘helper’ and directs it to ‘ans’ , and ensures that there is no extra space at the end.

4. Finally, the string with reversed words is stored in ‘ans’, and it is returned as a result.

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