# Find missing elements from an Array

Given a list of integers from the range [1, N] with some of the elements missing. The task is to find the missing elements. Note that there can be duplicates in the list.

Examples:

```Input: arr[] = {1, 3, 3, 3, 5}
Output: 2 4

Input: arr[] = {1, 2, 3, 4, 4, 7, 7}
Output: 5 6
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: In the given range [1, N] there should be an element corresponding to each index. If an element is missing then its index will never be visited.

```Traverse the array:
For each element:
if array[element] > 0:
Mark the element as visited
Again, traverse the array:
if element isNot Visited:
```

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach ` `#include ` `using` `namespace` `std; ` ` `  `// Function to find the missing elements ` `vector<``int``> missing_elements(vector<``int``> vec) ` `{ ` ` `  `    ``// Vector to store the list ` `    ``// of missing elements ` `    ``vector<``int``> mis; ` ` `  `    ``// For every given element ` `    ``for` `(``int` `i = 0; i < vec.size(); i++) { ` ` `  `        ``// Find its index ` `        ``int` `temp = ``abs``(vec[i]) - 1; ` ` `  `        ``// Update the element at the found index ` `        ``vec[temp] = vec[temp] > 0 ? -vec[temp] : vec[temp]; ` `    ``} ` `    ``for` `(``int` `i = 0; i < vec.size(); i++) ` ` `  `        ``// Current element was not present ` `        ``// in the original vector ` `        ``if` `(vec[i] > 0) ` `            ``mis.push_back(i + 1); ` ` `  `    ``return` `mis; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``vector<``int``> vec = { 3, 3, 3, 5, 1 }; ` ` `  `    ``// Vector to store the returned ` `    ``// list of missing elements ` `    ``vector<``int``> miss_ele = missing_elements(vec); ` ` `  `    ``// Print the list of elements ` `    ``for` `(``int` `i = 0; i < miss_ele.size(); i++) ` `        ``cout << miss_ele[i] << ``" "``; ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java implementation of the above approach  ` `import` `java.util.*; ` ` `  `class` `GFG ` `{ ` `     `  `    ``// Function to find the missing elements  ` `    ``static` `Vector missing_elements(Vector vec)  ` `    ``{  ` `     `  `        ``// Vector to store the list  ` `        ``// of missing elements  ` `        ``Vector mis = ``new` `Vector(); ` `     `  `        ``// For every given element  ` `        ``for` `(``int` `i = ``0``; i < vec.size(); i++)  ` `        ``{  ` `     `  `            ``// Find its index  ` `            ``int` `temp = Math.abs((``int``)vec.get(i)) - ``1``;  ` `     `  `            ``// Update the element at the found index  ` `            ``if` `((``int``)vec.get(temp) > ``0``) ` `                ``vec.set(temp,-(``int``)vec.get(temp)); ` `            ``else` `                ``vec.set(temp,vec.get(temp)); ` `        ``}  ` `        ``for` `(``int` `i = ``0``; i < vec.size(); i++)  ` `        ``{ ` `            ``// Current element was not present  ` `            ``// in the original vector  ` `            ``if` `((``int``)vec.get(i) > ``0``)  ` `                ``mis.add(i + ``1``);  ` `        ``} ` `        ``return` `mis;  ` `    ``}  ` `     `  `    ``// Driver code  ` `    ``public` `static` `void` `main(String args[])  ` `    ``{  ` `        ``Vector vec = ``new` `Vector();  ` `        ``vec.add(``3``); ` `        ``vec.add(``3``); ` `        ``vec.add(``3``); ` `        ``vec.add(``5``); ` `        ``vec.add(``1``); ` `         `  `        ``// Vector to store the returned  ` `        ``// list of missing elements  ` `        ``Vector miss_ele = missing_elements(vec);  ` `     `  `        ``// Print the list of elements  ` `        ``for` `(``int` `i = ``0``; i < miss_ele.size(); i++)  ` `            ``System.out.print(miss_ele.get(i) + ``" "``);  ` `    ``}  ` `} ` ` `  `// This code is contributed by AnkitRai01 `

## Python3

 `# Python3 implementation of the approach ` ` `  `# Function to find the missing elements ` `def` `missing_elements(vec): ` ` `  `    ``# Vector to store the list ` `    ``# of missing elements ` `    ``mis ``=` `[] ` ` `  `    ``# For every given element ` `    ``for` `i ``in` `range``(``len``(vec)): ` ` `  `        ``# Find its index ` `        ``temp ``=` `abs``(vec[i]) ``-` `1` ` `  `        ``# Update the element at the found index ` `        ``if` `vec[temp] > ``0``: ` `            ``vec[temp] ``=` `-``vec[temp] ` ` `  `    ``for` `i ``in` `range``(``len``(vec)): ` ` `  `        ``# Current element was not present ` `        ``# in the original vector ` `        ``if` `(vec[i] > ``0``): ` `            ``mis.append(i ``+` `1``) ` ` `  `    ``return` `mis ` ` `  `# Driver code ` `vec ``=` `[``3``, ``3``, ``3``, ``5``, ``1``] ` ` `  `# Vector to store the returned ` `# list of missing elements ` `miss_ele ``=` `missing_elements(vec) ` ` `  `# Print the list of elements ` `for` `i ``in` `range``(``len``(miss_ele)): ` `    ``print``(miss_ele[i], end ``=` `" "``) ` ` `  `# This code is contributed by Mohit Kumar `

## C#

 `// C# implementation of the approach ` `using` `System; ` `using` `System.Collections.Generic;              ` ` `  `class` `GFG ` `{ ` `     `  `    ``// Function to find the missing elements  ` `    ``static` `List<``int``> missing_elements(List<``int``> vec)  ` `    ``{  ` `     `  `        ``// List to store the list  ` `        ``// of missing elements  ` `        ``List<``int``> mis = ``new` `List<``int``>(); ` `     `  `        ``// For every given element  ` `        ``for` `(``int` `i = 0; i < vec.Count; i++)  ` `        ``{  ` `     `  `            ``// Find its index  ` `            ``int` `temp = Math.Abs((``int``)vec[i]) - 1;  ` `     `  `            ``// Update the element at the found index  ` `            ``if` `((``int``)vec[temp] > 0) ` `                ``vec[temp] = -(``int``)vec[temp]; ` `            ``else` `                ``vec[temp] = vec[temp]; ` `        ``}  ` `        ``for` `(``int` `i = 0; i < vec.Count; i++)  ` `        ``{ ` `            ``// Current element was not present  ` `            ``// in the original vector  ` `            ``if` `((``int``)vec[i] > 0)  ` `                ``mis.Add(i + 1);  ` `        ``} ` `        ``return` `mis;  ` `    ``}  ` `     `  `    ``// Driver code  ` `    ``public` `static` `void` `Main(String []args)  ` `    ``{  ` `        ``List<``int``> vec = ``new` `List<``int``>();  ` `        ``vec.Add(3); ` `        ``vec.Add(3); ` `        ``vec.Add(3); ` `        ``vec.Add(5); ` `        ``vec.Add(1); ` `         `  `        ``// List to store the returned  ` `        ``// list of missing elements  ` `        ``List<``int``> miss_ele = missing_elements(vec);  ` `     `  `        ``// Print the list of elements  ` `        ``for` `(``int` `i = 0; i < miss_ele.Count; i++)  ` `            ``Console.Write(miss_ele[i] + ``" "``);  ` `    ``}  ` `} ` ` `  `// This code is contributed by 29AjayKumar `

Output:

```2 4
```

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