Skip to content

Reverse Words in a String

Jump to bottom
Raymond Chen edited this page Apr 17, 2023 · 7 revisions

Problem Highlights

1: U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

  • Established a set (2-3) of test cases to verify their own solution later.
  • Established a set (1-2) of edge cases to verify their solution handles complexities.
  • Have fully understood the problem and have no clarifying questions.
  • Have you verified any Time/Space Constraints for this problem?
  • Could the input parameter be Null?
    • Let’s assume no input will be Null. However, the input could be an empty string.
HAPPY CASE
Input: "the sky is blue"
Output: "blue is the sky"

Input: "what is the time"
Output: "time the is what"

EDGE CASE (Multiple Spaces)
Input: "the   sky  is   blue"
Output: "blue is the sky"

2: M-atch

Match what this problem looks like to known categories of problems, e.g. Linked List or Dynamic Programming, and strategies or patterns in those categories.

  • Sort
    • Does sorting help us achieve what we need in order to solve the problem?
  • Two pointer solutions (left and right pointer variables)
    • Two pointer may help us tokenize the object if we are not allowed to use built-in language methods.
  • Storing the elements of the array in a HashMap or a Set
    • In reversing, hashing elements and storing them may not yield an optimal solution.
  • Traversing the array with a sliding window
    • Will viewing pieces of the input at a time help us?

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Tokenize the input string into words separated by spaces and rejoin the tokens in reversed order.

1) Tokenize the input string to create a separate array 
2) Return a joined string version of the reversed array

⚠️ Common Mistakes

  • You may have a hard time understanding this problem because most questions are the standard reverse a string with a single word problem.
  • Some people get stuck on solving the multiple spaces sub-problem.

4: I-mplement

Implement the code to solve the algorithm.

class Solution:
    def reverseWords(self, s: str) -> str:
        # Tokenize the input string to create a separate array 
        arr = [token for token in s.split() if token != ""]
        
        # Return a joined string version of the reversed array
        return " ".join(reversed(arr))
class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        // Tokenize the input string to create a separate array 
        String[] array = s.split(" ");

        // Return a joined string version of the reversed array
        for (int i = array.length - 1; i >= 0; i--) {
            if (!array[i].isEmpty()) {
                sb.append(array[i]);
                sb.append(" ");
            }
        }

        return sb.toString().trim();
    }
}

5: R-eview

Review the code by running specific example(s) and recording values (watchlist) of your code's variables along the way.

  • Trace through your code with an input to check for the expected output
  • Catch possible edge cases and off-by-one errors

6: E-valuate

Evaluate the performance of your algorithm and state any strong/weak or future potential work.

Assume N represents the number of character in the string.

  • Time Complexity: O(N), traversing done on every word in string
  • Space Complexity: O(N), we will be building a string with the length of the string in reverse.
Clone this wiki locally