C++ Program to Sort Characters in String

You can sort characters in a string in C++ by using std::sort(str.begin(), str.end()) which rearranges the characters in ascending order.

Examples

Inputstring

Outputginrst

Inputhello

Outputehllo

Inputa

Outputa

How to Think About It

To sort characters in a string, think of the string as a list of letters. You want to rearrange these letters from smallest to largest based on their order in the alphabet. Using a sorting function, you compare letters and swap them until the whole string is ordered.

Algorithm

1

Get the input string from the user.

2

Use a sorting method to reorder the characters in the string.

3

Print the sorted string as output.

Code

cpp

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

int main() {
    std::string str;
    std::cin >> str;
    std::sort(str.begin(), str.end());
    std::cout << str << std::endl;
    return 0;
}

Output

ehllo

Dry Run

Let's trace the input "hello" through the code

1

Input string

str = "hello"

2

Sort characters

After std::sort, str = "ehllo"

3

Print output

Output: ehllo

Index	Before Sort	After Sort
0	h	e
1	e	h
2	l	l
3	l	l
4	o	o

Why This Works

Step 1: Using std::sort

The std::sort function rearranges elements between two iterators in ascending order.

Step 2: String iterators

We use str.begin() and str.end() to tell std::sort which part of the string to sort.

Step 3: In-place sorting

The sorting happens inside the original string, so no extra string is needed.

Alternative Approaches

Using a vector and std::sort

cpp

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

int main() {
    std::string str;
    std::cin >> str;
    std::vector<char> chars(str.begin(), str.end());
    std::sort(chars.begin(), chars.end());
    for (char c : chars) std::cout << c;
    std::cout << std::endl;
    return 0;
}

This method uses extra memory for the vector but is useful if you want to manipulate characters separately.

Using counting sort for only lowercase letters

cpp

#include <iostream>
#include <string>

int main() {
    std::string str;
    std::cin >> str;
    int count[26] = {0};
    for (char c : str) count[c - 'a']++;
    for (int i = 0; i < 26; i++) {
        for (int j = 0; j < count[i]; j++) std::cout << char('a' + i);
    }
    std::cout << std::endl;
    return 0;
}

This is faster for strings with only lowercase letters but less flexible.

Complexity: O(n log n) time, O(1) space

Time Complexity

The sorting function std::sort uses a fast sorting algorithm (usually introsort) which runs in O(n log n) time where n is the string length.

Space Complexity

Sorting is done in-place on the string, so extra space is O(1).

Which Approach is Fastest?

Using std::sort is generally fastest and simplest. Counting sort is faster only for limited character sets like lowercase letters.

Approach	Time	Space	Best For
std::sort on string	O(n log n)	O(1)	General strings with any characters
Vector + std::sort	O(n log n)	O(n)	When needing separate character manipulation
Counting sort	O(n)	O(1)	Strings with only lowercase letters