C++ Program to Reverse Words in a String

Use C++ code like std::istringstream iss(str); std::vector words; while (iss >> word) words.push_back(word); std::reverse(words.begin(), words.end()); to split the string into words, reverse their order, and join them back.

Examples

Inputhello world

Outputworld hello

InputC++ is fun

Outputfun is C++

Inputsingle

Outputsingle

How to Think About It

To reverse words in a string, first split the string into individual words by spaces. Then reverse the order of these words. Finally, join the reversed words back into a single string separated by spaces.

Algorithm

1

Get the input string.

2

Split the string into words using spaces as separators.

3

Reverse the order of the words.

4

Join the reversed words into a single string separated by spaces.

5

Return or print the reversed string.

Code

cpp

#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>

int main() {
    std::string str = "C++ is fun";
    std::istringstream iss(str);
    std::vector<std::string> words;
    std::string word;

    while (iss >> word) {
        words.push_back(word);
    }

    std::reverse(words.begin(), words.end());

    for (size_t i = 0; i < words.size(); ++i) {
        std::cout << words[i];
        if (i != words.size() - 1) std::cout << " ";
    }
    std::cout << std::endl;
    return 0;
}

Dry Run

Let's trace the input "C++ is fun" through the code.

1

Input string

str = "C++ is fun"

2

Split into words

words = ["C++", "is", "fun"]

3

Reverse words

words = ["fun", "is", "C++"]

4

Join and print

Output: "fun is C++"

Iteration	word	words vector
1	C++	["C++"]
2	is	["C++", "is"]
3	fun	["C++", "is", "fun"]

Why This Works

Step 1: Splitting the string

Using std::istringstream and extraction operator >> splits the string into words separated by spaces.

Step 2: Reversing the words

The std::reverse function reverses the order of words in the vector.

Step 3: Joining and printing

A loop prints the reversed words separated by spaces to form the final reversed string.

Alternative Approaches

Using stack

cpp

#include <iostream>
#include <sstream>
#include <stack>

int main() {
    std::string str = "hello world";
    std::istringstream iss(str);
    std::stack<std::string> stk;
    std::string word;

    while (iss >> word) {
        stk.push(word);
    }

    while (!stk.empty()) {
        std::cout << stk.top();
        stk.pop();
        if (!stk.empty()) std::cout << " ";
    }
    std::cout << std::endl;
    return 0;
}

Uses a stack to reverse words, which is intuitive but may use more memory than vector reverse.

In-place reversal of characters

cpp

#include <iostream>
#include <algorithm>
#include <string>

void reverseWords(std::string &s) {
    std::reverse(s.begin(), s.end());
    size_t start = 0, end = 0;
    while ((end = s.find(' ', start)) != std::string::npos) {
        std::reverse(s.begin() + start, s.begin() + end);
        start = end + 1;
    }
    std::reverse(s.begin() + start, s.end());
}

int main() {
    std::string s = "C++ is fun";
    reverseWords(s);
    std::cout << s << std::endl;
    return 0;
}

Reverses entire string then each word to reverse word order in-place, saving extra space.

Complexity: O(n) time, O(n) space

Time Complexity

Splitting the string and reversing the words each take linear time proportional to the string length n.

Space Complexity

Extra space is used to store words in a vector, which can be up to O(n) in size.

Which Approach is Fastest?

In-place reversal uses O(1) extra space but is more complex; vector or stack methods are simpler but use O(n) space.

Approach	Time	Space	Best For
Vector + reverse	O(n)	O(n)	Simplicity and readability
Stack	O(n)	O(n)	Intuitive reversal using LIFO
In-place reversal	O(n)	O(1)	Memory efficiency and in-place modification