Remove Duplicate Characters from a String

Code

#include <stdio.h>
#include <stdint.h>

void remove_duplicates(char *str) {
    uint8_t i = 0, j = 0;
    char seen[256] = {0};  // ASCII character map

    while (str[i]) {
        if (!seen[(uint8_t)str[i]]) {
            seen[(uint8_t)str[i]] = 1;
            str[j++] = str[i];
        }
        i++;
    }
    str[j] = '\0';  // Null-terminate the result
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    uint8_t i = 0;
    while (str[i]) {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    remove_duplicates(str);
    printf("%s", str);
    return 0;
}

Solving Approach

1. Track seen characters

• Use a fixed-size array seen[256] to represent all possible ASCII characters.
• Each index corresponds to a character (seen['e'], seen['m'], etc.).
• Initialize all entries to 0.

2. Iterate through the input string

• Use two pointers:
• i: scans the original string
• j: builds the result in-place

3. Check if character is already seen

• If seen[str[i]] == 0, it’s the first time we’ve seen this character:
  • Set seen[str[i]] = 1
  • Copy str[i] to str[j]
  • Increment j
• If seen[str[i]] == 1, skip it.

4. Terminate the result

• After the loop, set str[j] = '\0' to properly end the string.

🧩 Embedded-Friendly Notes

• No dynamic memory
• No string libraries
• Works in-place — no extra string buffer
• Uses only char, uint8_t, and fixed-size arrays