Remove Duplicate Values

#include <stdio.h>
int main(void) {
    int input[] = {1, 2, 1, 3, 2, 4}, output[6], size = 0;
    // Keep the first occurrence of every value.
    for (int i = 0; i < 6; ++i) { int seen = 0; for (int j = 0; j < size; ++j) if (output[j] == input[i]) seen = 1; if (!seen) output[size++] = input[i]; }
    for (int i = 0; i < size; ++i) printf("%s%d", i ? " " : "", output[i]); puts("");
}

#include <iostream>
#include <unordered_set>
#include <vector>
int main() {
    std::vector input{1, 2, 1, 3, 2, 4}; std::vector<int> output; std::unordered_set<int> seen;
    // Insert returns true only for the first occurrence.
    for (int value : input) if (seen.insert(value).second) output.push_back(value);
    for (size_t i = 0; i < output.size(); ++i) std::cout << (i ? " " : "") << output[i]; std::cout << '\n';
}