Reverse words in a given string | Set 2
Given string str, the task is to reverse the string by considering each word of the string, str as a single unit.
Examples:
Input: str = “Beginner quiz practice code”
Output: code practice quiz Beginner
Explanation:
The words in the given string are [“Beginner”, “quiz”, “practice”, “code”].
Therefore, after reversing the order of the words, the required output is“code practice quiz Beginner”.Input: str = “getting good at coding needs a lot of practice”
Output: practice of lot a needs coding at good getting
In-place Reversal Approach: Refer to the article Reverse words in a given string for the in-place reversal of words followed by a reversal of the entire string.
Time Complexity: O(N)
Auxiliary Space: O(1)
Stack-based Approach: In this article, the approach to solving the problem using Stack is going to be discussed. The idea here is to push all the words of str into the Stack and then print all the elements of the Stack. Follow the steps below to solve the problem:
- Create a Stack to store each word of the string str.
- Iterate over string str, and separate each word of str by a space delimiter.
- Push all the words of str into the stack.
- Print all the elements of the stack one by one.
Below is the implementation of the above approach:
C++
// C++ Program to implement // the above approach #include <bits/stdc++.h> using namespace std; // Function to reverse the words // of a given string void printRev(string str) { // Stack to store each // word of the string stack<string> st; // Store the whole string // in string stream stringstream ss(str); string temp; while (getline(ss, temp, ' ' )) { // Push each word of the // string into the stack st.push(temp); } // Print the string in reverse // order of the words while (!st.empty()) { cout << st.top() << " " ; st.pop(); } } // Driver Code int main() { string str; str = "Beginner quiz practice code" ; printRev(str); return 0; } |
Java
// Java Program to implement // the above approach import java.util.*; class GFG{ // Function to reverse the words // of a given String static void printRev(String str) { // Stack to store each // word of the String Stack<String> st = new Stack<String>(); // Store the whole String // in String stream String[] ss = str.split( " " ); for (String temp : ss) { // Push each word of the // String into the stack st.add(temp); } // Print the String in reverse // order of the words while (!st.isEmpty()) { System.out.print(st.peek() + " " ); st.pop(); } } // Driver Code public static void main(String[] args) { String str; str = "Beginner quiz practice code" ; printRev(str); } } // This code is contributed by Rajput-Ji |
Python3
# Python3 program to implement # the above approach # Function to reverse the words # of a given string def printRev(strr): # Stack to store each # word of the string strr = strr.split( " " ) st = [] # Store the whole string # in stream for i in strr: # Push each word of the # into the stack st.append(i) # Print the in reverse # order of the words while len (st) > 0 : print (st[ - 1 ], end = " " ) del st[ - 1 ] # Driver Code if __name__ = = '__main__' : strr = "Beginner quiz practice code" printRev(strr) # This code is contributed by mohit kumar 29 |
C#
// C# program to implement // the above approach using System; using System.Collections; class GFG{ // Function to reverse the words // of a given String static void printRev( string str) { // Stack to store each // word of the String Stack st = new Stack(); String[] separator = { " " }; // Store the whole String // in String stream string [] ss = str.Split(separator, int .MaxValue, StringSplitOptions.RemoveEmptyEntries); foreach ( string temp in ss) { // Push each word of the // String into the stack st.Push(temp); } // Print the String in reverse // order of the words while (st.Count > 0) { Console.Write(st.Peek() + " " ); st.Pop(); } } // Driver Code public static void Main( string [] args) { string str; str = "Beginner quiz practice code" ; printRev(str); } } // This code is contributed by rutvik_56 |
Javascript
<script> // Javascript Program to implement // the above approach // Function to reverse the words // of a given String function printRev(str) { // Stack to store each // word of the String let st = []; // Store the whole String // in String stream let ss = str.split( " " ); for (let temp=0;temp< ss.length;temp++) { // Push each word of the // String into the stack st.push(ss[temp]); } // Print the String in reverse // order of the words while (st.length!=0) { document.write(st.pop() + " " ); } } // Driver Code let str; str = "Beginner quiz practice code" ; printRev(str); // This code is contributed by unknown2108 </script> |
code practice quiz Beginner
Time Complexity: O(N), where N denotes the length of the string.
Auxiliary Space: O(N)
Method #2: Using built-in python functions
- As all the words in a sentence are separated by spaces.
- We have to split the sentence by spaces using split().
- We split all the words by spaces and store them in a list.
- Reverse this list and print it.
C++
// C++ program to implement // the above approach #include <bits/stdc++.h> using namespace std; // Function to reverse the words // of a given string void printRev(string lis[]) { // reverse the list reverse(lis, lis + 4); for ( int i = 0; i < 4; i++) { cout << lis[i] << " " ; } } int main() { string strr[] = { "Beginner" , "quiz" , "practice" , "code" }; printRev(strr); return 0; } // This code is contributed by divyeshrabadiya07. |
Java
// Java program to implement // the above approach import java.util.*; class GFG{ // Function to reverse the words // of a given string static void printRev(String string) { // Split by space and converting // string to list String[] lis = string.split( " " , 0 ); // Reverse the list Collections.reverse(Arrays.asList(lis)); // Printing the list for (String li : lis) { System.out.print(li + " " ); } } // Driver code public static void main(String[] args) { String strr = "Beginner quiz practice code" ; printRev(strr); } } // This code is contributed by decode2207 |
Python3
# Python3 program to implement # the above approach # Function to reverse the words # of a given string def printRev(string): # split by space and converting # string to list lis = list (string.split()) # reverse the list lis.reverse() # printing the list print ( * lis) # Driver Code if __name__ = = '__main__' : strr = "Beginner quiz practice code" printRev(strr) # This code is contributed by vikkycirus |
C#
// C# program to implement // the above approach using System; class GFG { // Function to reverse the words // of a given string static void printRev( string String) { // split by space and converting // string to list string [] lis = String.Split( ' ' ); // reverse the list Array.Reverse(lis); // printing the list foreach ( string li in lis) { Console.Write(li + " " ); } } static void Main() { string strr = "Beginner quiz practice code" ; printRev(strr); } } // This code is contributed by rameshtravel07. |
Javascript
<script> // javascript program to implement // the above approach // Function to reverse the words // of a given string function printRev(string){ // split by space and converting // string to list var lis = string.split( ' ' ); console.log(lis); // reverse the list lis.reverse(); console.log(lis); // printing the list document.write(lis.join( ' ' )); } // Driver Code var strr = "Beginner quiz practice code" printRev(strr) </script> |
code practice quiz Beginner
Time Complexity : O(n), n is the number of strings.
Auxiliary Space : O(1)
Using a loop and stack in python:
Approach:
The function starts by importing the time module, which is used to measure the time taken to perform the operation.
The function defines a variable stack as an empty list, which will be used to store the words in reverse order.
The function also defines a variable word as an empty string, which will be used to store each word in the input string as it is being read.
The function then iterates over each character c in the input string s.
If the character is a space, the word variable is appended to the stack list, and the word variable is reset to an empty string to start building the next word.
If the character is not a space, it is added to the current word being built.
After all characters have been processed, the last word variable is appended to the stack list.
The function then defines a variable result as an empty string, which will be used to store the reversed string.
The function then enters a loop that pops words from the stack list in reverse order, concatenates them with a space, and adds the resulting string to the result variable.
The loop continues until all words have been popped from the stack list.
The resulting string in result is stripped of any trailing spaces and returned along with the time taken to perform the operation.
Finally, the function is called twice with two different input strings (s1 and s2) to demonstrate how the function works.
Python3
import time def reverse_words(s): start_time = time.time() stack = [] word = "" for c in s: if c = = " " : stack.append(word) word = "" else : word + = c stack.append(word) result = "" while stack: result + = stack.pop() + " " result = result.strip() end_time = time.time() return result, end_time - start_time # example usage s1 = "Beginner quiz practice code" s2 = "getting good at coding needs a lot of practice" result, time_taken = reverse_words(s1) print (result) print (f "Time taken: {time_taken} seconds" ) result, time_taken = reverse_words(s2) print (result) print (f "Time taken: {time_taken} seconds" ) |
code practice quiz Beginner Time taken: 1.049041748046875e-05 seconds practice of lot a needs coding at good getting Time taken: 6.9141387939453125e-06 seconds
Time complexity: O(n), where n is the length of the input string. This is because we iterate over each character in the string once, which takes O(n) time. We also append each word to a stack, which takes O(1) time per operation.
Space complexity: O(n), where n is the length of the input string. This is because we create a stack to store the words, which can take up to O(n) space in the worst case.
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