Algorithm Approach

Using a boolean lookup array representing the 256 possible ASCII character codes, we can track seen characters in O(1) time and modify the string in-place with a write pointer:

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Complete C Code

c (ISO Standard)
Run in Compiler.cCopy
#include <stdio.h>
#include <stdbool.h>

void removeDuplicates(char *str) {
    if (str == NULL) return;

    bool seen[256] = {false};
    int readIdx = 0;
    int writeIdx = 0;

    while (str[readIdx] != '\0') {
        unsigned char ch = (unsigned char)str[readIdx];

        if (!seen[ch]) {
            seen[ch] = true;
            str[writeIdx++] = str[readIdx];
        }
        readIdx++;
    }

    str[writeIdx] = '\0'; /* Null terminate the modified string */
}

int main() {
    char sample[] = "programming in c language";

    printf("Original String: \"%s\"\n", sample);
    removeDuplicates(sample);
    printf("Without Duplicates: \"%s\"\n", sample);

    return 0;
}

Sample Output

text (ISO Standard)
Copy
Original String: "programming in c language"
Without Duplicates: "progamin c lue"

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Complexity Analysis

• Time Complexity: O(n) single-pass linear time.
• Auxiliary Space: O(1) (Fixed 256-byte ASCII array).

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Related Programs

• Character analysis in Count Vowels and Consonants in C.
• Master array allocations in Arrays in C Programming.