Related Articles
Longest Subarray consisiting of unique elements from an Array
• Difficulty Level : Hard
• Last Updated : 21 Aug, 2020

Given an array arr[] consisting of N integers, the task is to find the largest subarray consisting of unique elements only.

Examples:

Input: arr[] = {1, 2, 3, 4, 5, 1, 2, 3}
Output:
Explanation: One possible subarray is {1, 2, 3, 4, 5}.

Input: arr[]={1, 2, 4, 4, 5, 6, 7, 8, 3, 4, 5, 3, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4}
Output:
Explanation: Only possible subarray is {3, 4, 5, 6, 7, 8, 1, 2}.

Naive Approach: The simplest approach to solve the problem is to generate all subarrays from the given array and check if it contains any duplicates or not using HashSet. Find the longest subarray satisfying the condition.

Time Complexity: O(N3logN)
Auxiliary Space: O(N)

Efficient Approach: The above approach can be optimised using HashMap. Follow the steps below to solve the problem:

1. Initialize a variable j, to store the maximum value of the index such that there is no repeated elements between index i and j
2. Traverse the array and keep updating j based on previous occurrence of a[i[ stored in the HashMap.
3. After updating j, update ans accordingly to store maximum length of desired subarray.
4. Print ans, after traversal is completed.

Below is the implementation of above approach:

## C++

 `// C++ program to implement``// the above approach``#include ``using` `namespace` `std;`` ` `// Function to find largest``// subarray with no dublicates``int` `largest_subarray(``int` `a[], ``int` `n)``{``    ``// Stores index of array elements``    ``unordered_map<``int``, ``int``> index;``    ``int` `ans = 0;``    ``for` `(``int` `i = 0, j = 0; i < n; i++) {`` ` `        ``// Update j based on previous``        ``// occurrence of a[i]``        ``j = max(index[a[i]], j);`` ` `        ``// Update ans to store maximum``        ``// length of subarray``        ``ans = max(ans, i - j + 1);`` ` `        ``// Store the index of current``        ``// occurrence of a[i]``        ``index[a[i]] = i + 1;``    ``}`` ` `    ``// Return final ans``    ``return` `ans;``}`` ` `// Driver Code``int32_t main()``{``    ``int` `arr[] = { 1, 2, 3, 4, 5, 1, 2, 3 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``cout << largest_subarray(arr, n);``}`

## Java

 `// Java program to implement``// the above approach``import` `java.util.*;``class` `GFG{`` ` `// Function to find largest``// subarray with no dublicates``static` `int` `largest_subarray(``int` `a[], ``int` `n)``{``    ``// Stores index of array elements``    ``HashMap index = ``new` `HashMap();``    ``int` `ans = ``0``;``    ``for``(``int` `i = ``0``, j = ``0``; i < n; i++)``    ``{`` ` `        ``// Update j based on previous``        ``// occurrence of a[i]``        ``j = Math.max(index.containsKey(a[i]) ? ``                             ``index.get(a[i]) : ``0``, j);`` ` `        ``// Update ans to store maximum``        ``// length of subarray``        ``ans = Math.max(ans, i - j + ``1``);`` ` `        ``// Store the index of current``        ``// occurrence of a[i]``        ``index.put(a[i], i + ``1``);``    ``}`` ` `    ``// Return final ans``    ``return` `ans;``}`` ` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``1``, ``2``, ``3``, ``4``, ``5``, ``1``, ``2``, ``3` `};``    ``int` `n = arr.length;``    ``System.out.print(largest_subarray(arr, n));``}``}`` ` `// This code is contributed by Rajput-Ji`

## Python3

 `# Python3 program to implement``# the above approach``from` `collections ``import` `defaultdict`` ` `# Function to find largest``# subarray with no dublicates``def` `largest_subarray(a, n):`` ` `    ``# Stores index of array elements``    ``index ``=` `defaultdict(``lambda` `: ``0``)``     ` `    ``ans ``=` `0``    ``j ``=` `0`` ` `    ``for` `i ``in` `range``(n):`` ` `        ``# Update j based on previous``        ``# occurrence of a[i]``        ``j ``=` `max``(index[a[i]], j)`` ` `        ``# Update ans to store maximum``        ``# length of subarray``        ``ans ``=` `max``(ans, i ``-` `j ``+` `1``)`` ` `        ``# Store the index of current``        ``# occurrence of a[i]``        ``index[a[i]] ``=` `i ``+` `1`` ` `        ``i ``+``=` `1`` ` `    ``# Return final ans ``    ``return` `ans`` ` `# Driver Code``arr ``=` `[ ``1``, ``2``, ``3``, ``4``, ``5``, ``1``, ``2``, ``3` `]``n ``=` `len``(arr)`` ` `# Function call``print``(largest_subarray(arr, n))`` ` `# This code is contributed by Shivam Singh`

## C#

 `// C# program to implement``// the above approach``using` `System;``using` `System.Collections.Generic;`` ` `class` `GFG{`` ` `// Function to find largest``// subarray with no dublicates``static` `int` `largest_subarray(``int` `[]a, ``int` `n)``{``     ` `    ``// Stores index of array elements``    ``Dictionary<``int``,``               ``int``> index = ``new` `Dictionary<``int``,``                                           ``int``>();``    ``int` `ans = 0;``    ``for``(``int` `i = 0, j = 0; i < n; i++)``    ``{`` ` `        ``// Update j based on previous``        ``// occurrence of a[i]``        ``j = Math.Max(index.ContainsKey(a[i]) ? ``                                 ``index[a[i]] : 0, j);`` ` `        ``// Update ans to store maximum``        ``// length of subarray``        ``ans = Math.Max(ans, i - j + 1);`` ` `        ``// Store the index of current``        ``// occurrence of a[i]``        ``if``(index.ContainsKey(a[i]))``            ``index[a[i]] = i + 1;``        ``else``            ``index.Add(a[i], i + 1);``    ``}`` ` `    ``// Return readonly ans``    ``return` `ans;``}`` ` `// Driver Code``public` `static` `void` `Main(String[] args)``{``    ``int` `[]arr = { 1, 2, 3, 4, 5, 1, 2, 3 };``    ``int` `n = arr.Length;``     ` `    ``Console.Write(largest_subarray(arr, n));``}``}`` ` `// This code is contributed by Amit Katiyar`
Output:
```5
```

Time Complexity: O(NlogN)
Auxiliary Space: O(N)

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

My Personal Notes arrow_drop_up