Remove Duplicates from String - Problem

Given a string, remove all duplicate characters while preserving the order of first appearance.

For example, if the input string is "programming", the output should be "progamin" because we keep the first occurrence of each character and remove subsequent duplicates.

Note: The comparison is case-sensitive, so 'A' and 'a' are considered different characters.

Input & Output

Example 1 — Basic Case

$ Input: s = "programming"

› Output: "proga"

💡 Note: Keep first occurrence of each character: p(1st), r(1st), o(1st), g(1st), r(duplicate-skip), a(1st), m(1st), m(duplicate-skip), i(1st), n(1st), g(duplicate-skip) → "progamin"

Example 2 — All Unique

$ Input: s = "abcdef"

› Output: "abcdef"

💡 Note: No duplicates found, so the output is the same as input

Example 3 — All Same

$ Input: s = "aaaa"

› Output: "a"

💡 Note: Only keep the first 'a', remove the three duplicates

Constraints

1 ≤ s.length ≤ 1000
s contains only printable ASCII characters

Visualization

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Asked in

G Google 35 M Microsoft 28 a Amazon 22 f Meta 18

The key insight is to use a hash set to track characters we've already encountered. The optimal approach processes the string in a single pass with O(n) time complexity. Hash Set approach: Time O(n), Space O(k) where k is the number of unique characters.

Common Approaches

✓ Brute Force - Check Each Previous Character

⏱️ Time: O(n²) Space: O(1)

Iterate through the string and for each character, scan all previous characters to check if it has appeared before. If not found, add it to the result string.

Hash Set - Track Seen Characters

⏱️ Time: O(n) Space: O(k)

Iterate through the string once, using a hash set to track which characters we've encountered. Only add characters to result if they haven't been seen before.

Brute Force - Check Each Previous Character — Algorithm Steps

Iterate through each character in the string
For each character, check all previous characters
If character hasn't appeared before, add it to result
Return the result string

Visualization

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Step-by-Step Walkthrough

Start with first char

No previous chars to check, add 'p'

Check 'r' against 'p'

Not duplicate, add 'r'

Result

Continue until all characters processed

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdbool.h>

void solution(char* s, char* result) {
    int len = strlen(s);
    int resultIndex = 0;
    
    for (int i = 0; i < len; i++) {
        bool isDuplicate = false;
        for (int j = 0; j < i; j++) {
            if (s[i] == s[j]) {
                isDuplicate = true;
                break;
            }
        }
        if (!isDuplicate) {
            result[resultIndex++] = s[i];
        }
    }
    result[resultIndex] = '\0';
}

int main() {
    char s[1001];
    char result[1001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = '\0';
    solution(s, result);
    printf("%s\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n²)

For each character, we scan all previous characters - creates nested loops

⚠ Quadratic Growth

Space Complexity

O(1)

Only using variables to store result, no extra data structures

✓ Linear Space

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Remove Duplicates from String - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force - Check Each Previous Character — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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