Question

How to find an element that appears once in a vector of elements that appears three times in C++

Answer 1

#include <unordered_map>
#include <iostream>
#include <vector>

int findElementThatAppearsOnceInArray(std::vector<int>& vec) {
    std::unordered_map<int, int> map;
        
    for (auto x: vec) {
        map[x]++;
    }

    for (auto x: map) {
        if (x.second == 1) {
            return x.first;
        }
    }
    
    return -1;
}


int main() {
    std::vector<int> vec {3, 5, 5, 2, 7, 3, 2, 8, 8, 3, 2, 5, 8};
         
    std::cout << findElementThatAppearsOnceInArray((vec));
}



/*
run:

7

*/

 

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Answer 2

#include <iostream>
#include <vector>

int findElementThatAppearsOnceInArray(std::vector<int>& vec) {
    int result = 0;

    for (int i = 0; i < 32; ++i) {
        int sum = 0;
        for (const int num : vec) {
            sum += num >> i & 1;
        }
        sum %= 3;
        result |= sum << i;
    }

    return result;
}

int main() {
    std::vector<int> vec {3, 5, 5, 2, 7, 3, 2, 8, 8, 3, 2, 5, 8};
         
    std::cout << findElementThatAppearsOnceInArray((vec));
}



/*
run:

7

*/

 

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