Find All Duplicates in an Array in C++

Suppose we have an array of integers, in range 1 ? a[i] ? n (n = size of an array), here some elements appear twice and others appear once. We have to find all the elements that appear twice in this array. So if the array is [4,3,2,7,8,2,3,1], then the output will be [2, 3].

Approach

To solve this, we will follow these steps ?

• n := size of array, make one array called ans
• for i in range 0 to n - 1
  • x := absolute value of nums[i]
  • decrease x by 1
  • if nums[x] < 0, then add x + 1 into ans, otherwise nums[x] := nums[x] * (-1)
• return ans

Example(C++)

Let us see the following implementation to get a better understanding ?

#include <bits/stdc++.h>
using namespace std;

// Function to print a vector
void print_vector(vector<int> v) {
    cout << "[";
    for (int i = 0; i < v.size(); i++) {
        cout << v[i];
        if (i != v.size() - 1) cout << ", ";
    }
    cout << "]" << endl;
}

class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        int n = nums.size();
        vector<int> ans;
        for (int i = 0; i < n; i++) {
            int x = abs(nums[i]);
            x--;
            if (nums[x] < 0)
                ans.push_back(x + 1);
            else
                nums[x] *= -1;
        }
        return ans;
    }
};

int main() {
    Solution ob;
    vector<int> v = {4, 3, 2, 7, 8, 2, 3, 1};
    print_vector(ob.findDuplicates(v));
    return 0;
}

Input

[4,3,2,7,8,2,3,1]

Output

[2,3]