Example: Find First Non-Repeating Character (Frequency Array) in C

// Find First Non-Repeating Character (Frequency Array)
#include <stdio.h>
#include <string.h> // For strlen

#define MAX_CHARS 256 // Assuming ASCII characters

int main() {
    char str[] = "aabbcdeff"; // Example string
    int len = strlen(str);
    int freq[MAX_CHARS] = {0}; // Initialize all frequencies to 0

    // Step 1: First pass - Populate the frequency array
    // Iterate through the string and count occurrences of each character
    for (int i = 0; i < len; i++) {
        // Increment count for the character at its ASCII value index
        freq[(unsigned char)str[i]]++; 
    }

    char firstNonRepeating = '\0'; // Initialize with null character

    // Step 2: Second pass - Find the first character with frequency 1
    // Iterate through the string again, in order, to find the first character
    // whose count in the freq array is 1.
    for (int i = 0; i < len; i++) {
        if (freq[(unsigned char)str[i]] == 1) {
            firstNonRepeating = str[i];
            break; // Found the first non-repeating, no need to continue
        }
    }

    // Step 3: Print the result
    if (firstNonRepeating != '\0') {
        printf("The first non-repeating character is: %c\n", firstNonRepeating);
    } else {
        printf("No non-repeating character found.\n");
    }

    return 0;
}