C++ Program to Find First Non Repeating Character

Use a C++ program that counts character frequencies with a std::unordered_map and then finds the first character with count 1 by iterating the string twice.

Examples

Inputswiss

Outputw

Inputhello

Outputh

Inputaabbcc

OutputNo non-repeating character found

How to Think About It

First, count how many times each character appears in the string. Then, look at the string from the start and find the first character that appears only once.

Algorithm

1

Get the input string.

2

Create a map to count each character's frequency.

3

Loop through the string and update counts in the map.

4

Loop through the string again and check the count of each character.

5

Return the first character with count 1 or indicate none found.

Code

cpp

#include <iostream>
#include <unordered_map>
#include <string>

int main() {
    std::string s;
    std::getline(std::cin, s);
    std::unordered_map<char, int> freq;

    for (char c : s) {
        freq[c]++;
    }

    for (char c : s) {
        if (freq[c] == 1) {
            std::cout << c << std::endl;
            return 0;
        }
    }

    std::cout << "No non-repeating character found" << std::endl;
    return 0;
}

Output

w

Dry Run

Let's trace the input "swiss" through the code.

1

Count characters

s:3, w:1, i:1

2

Find first non-repeating

Check 's' count=3 (skip), 'w' count=1 (found)

Character	Frequency
s	3
w	1
i	1

Why This Works

Step 1: Counting frequencies

We use unordered_map to store how many times each character appears, so we know which are repeated.

Step 2: Finding the first unique

By checking characters in original order, the first with count 1 is the first non-repeating character.

Alternative Approaches

Using array for frequency (only for ASCII)

cpp

#include <iostream>
#include <string>

int main() {
    std::string s;
    std::getline(std::cin, s);
    int freq[256] = {0};

    for (char c : s) {
        freq[(unsigned char)c]++;
    }

    for (char c : s) {
        if (freq[(unsigned char)c] == 1) {
            std::cout << c << std::endl;
            return 0;
        }
    }

    std::cout << "No non-repeating character found" << std::endl;
    return 0;
}

Faster for ASCII but not suitable for Unicode or large character sets.

Using two loops without extra space

cpp

#include <iostream>
#include <string>

int main() {
    std::string s;
    std::getline(std::cin, s);

    for (size_t i = 0; i < s.size(); ++i) {
        bool unique = true;
        for (size_t j = 0; j < s.size(); ++j) {
            if (i != j && s[i] == s[j]) {
                unique = false;
                break;
            }
        }
        if (unique) {
            std::cout << s[i] << std::endl;
            return 0;
        }
    }

    std::cout << "No non-repeating character found" << std::endl;
    return 0;
}

No extra memory but slower (O(n²)) for large strings.

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

Time Complexity

Counting frequencies and checking characters each take O(n), so total is O(n).

Space Complexity

We use extra space for frequency map proportional to unique characters k.

Which Approach is Fastest?

Using a hash map is fastest for general strings; array method is faster for ASCII but limited; two loops use no extra space but are slower.

Approach	Time	Space	Best For
Hash Map Frequency	O(n)	O(k)	General strings with any characters
Array Frequency	O(n)	O(1)	ASCII strings, faster but limited charset
Two Loops	O(n²)	O(1)	Small strings or memory-limited environments