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 = “geeks quiz practice code”
Output: code practice quiz geeks 
Explanation: 
The words in the given string are [“geeks”, “quiz”, “practice”,  “code”]. 
Therefore, after reversing the order of the words, the required output is“code practice quiz geeks”.

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:

1. Create a Stack to store each word of the string str.
2. Iterate over string str, and separate each word of str by a space delimiter.
3. Push all the words of str into the stack.
4. 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 = "geeks 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 = "geeks 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 = "geeks 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 = "geeks 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 = "geeks quiz practice code";
printRev(str);


// This code is contributed by unknown2108
</script>

Output

code practice quiz geeks 

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[] = {"geeks", "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 = "geeks 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 = "geeks 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 = "geeks 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 = "geeks quiz practice code"

    printRev(strr)

</script>

Output

code practice quiz geeks

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 = "geeks 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")

Output

code practice quiz geeks
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.