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 6Approach: 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:
add it as missing elementBelow is the implementation of the above approach:
C++
// C++ implementation of the approach#include <bits/stdc++.h>using namespace std;// Function to find the missing elementsvector<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 codeint 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 approachimport 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 elementsdef 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 codevec = [3, 3, 3, 5, 1]# Vector to store the returned# list of missing elementsmiss_ele = missing_elements(vec)# Print the list of elementsfor i in range(len(miss_ele)): print(miss_ele[i], end = " ")# This code is contributed by Mohit Kumar |
C#
// C# implementation of the approachusing System;using System.Collections.Generic; class GFG{ // Function to find the missing elements static List<int> missing_elements(List<int> vec) { // List<int> 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 |
Javascript
<script> // Javascript implementation of the approach // Function to find the missing elements function missing_elements(vec) { // Vector to store the list // of missing elements let mis = []; // For every given element for (let i = 0; i < vec.length; i++) { // Find its index let temp = Math.abs(vec[i]) - 1; // Update the element at the found index vec[temp] = vec[temp] > 0 ? -vec[temp] : vec[temp]; } for (let i = 0; i < vec.length; i++) // Current element was not present // in the original vector if (vec[i] > 0) mis.push(i + 1); return mis; } let vec = [ 3, 3, 3, 5, 1 ]; // Vector to store the returned // list of missing elements let miss_ele = missing_elements(vec); // Print the list of elements for (let i = 0; i < miss_ele.length; i++) document.write(miss_ele[i] + " ");</script> |
Output:
2 4
Time Complexity: O(N), where N is the length of the vector vec.
Auxiliary Space: O(N)
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