# Find All Duplicates in an Array in C++

Suppose we have an array of integers, in range 1 ≤ a[i] ≤ n (n = size of 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]

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 −

Live Demo

#include <bits/stdc++.h>
using namespace std;
void print_vector(vector<auto> v){
cout << "[";
for(int i = 0; i<v.size(); i++){
cout << v[i] << ", ";
}
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;
}
};
main(){
Solution ob;
vector<int> v = {4,3,2,7,8,2,3,1};
print_vector(ob.findDuplicates(v));
}

## Input

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

## Output

[2,3]