# Maximum difference between first and last indexes of an element in array

Given an array of n integers. The task is to find the difference of first and last index of each distinct element so as to maximize the difference.

Examples:

Input : {2, 1, 3, 4, 2, 1, 5, 1, 7} Output : 6 Element1has itsfirst index= 1 andlast index= 7Difference= 7 - 1 = 6 Other elements have a smaller first and last index difference Input : {2, 2, 1, 1, 8, 8, 3, 5, 3} Output : 2 Maximum difference is for indexes of element 3.

A **simple approach** is to run two loops and find the difference for each element and accordingly update the **max_diff**. It has a time complexity of O(n^{2}) and the approach also needs to keep track of the elements that have been visited so that difference for them is not calculated unnecessarily.

An **efficient approach** uses hashing. It has the following steps.

- Traverse the input array from left to right.
- For each distinct element map its first and last index in the hash table.
- Traverse the hash table and calculate the first and last index difference for each element.
- Accordingly update the
**max_diff**.

In the following implementation unordered_map has been used for hashing as the range of integers is not known.

## C++

`// C++ implementation to find the maximum difference ` `// of first and last index of array elements` `#include <bits/stdc++.h>` ` ` `using` `namespace` `std;` ` ` `// function to find the` `// maximum difference` `int` `maxDifference(` `int` `arr[], ` `int` `n)` `{` ` ` `// structure to store first and last` ` ` `// index of each distinct element` ` ` `struct` `index` ` ` `{` ` ` `int` `f, l;` ` ` `};` ` ` ` ` `// maps each element to its` ` ` `// 'index' structure` ` ` `unordered_map<` `int` `, index> um;` ` ` ` ` `for` `(` `int` `i=0; i<n; i++)` ` ` `{` ` ` `// storing first index` ` ` `if` `(um.find(arr[i]) == um.end())` ` ` `um[arr[i]].f = i;` ` ` ` ` `// storing last index ` ` ` `um[arr[i]].l = i; ` ` ` `}` ` ` ` ` `int` `diff, max_diff = INT_MIN;` ` ` ` ` `unordered_map<` `int` `, index>::iterator itr;` ` ` ` ` `// traversing 'um'` ` ` `for` `(itr=um.begin(); itr != um.end(); itr++)` ` ` `{ ` ` ` `// difference of last and first index` ` ` `// of each element` ` ` `diff = (itr->second).l - (itr->second).f;` ` ` ` ` `// update 'max_dff'` ` ` `if` `(max_diff < diff)` ` ` `max_diff = diff;` ` ` `}` ` ` ` ` `// required maximum difference` ` ` `return` `max_diff;` `}` ` ` ` ` `// Driver program to test above` `int` `main()` `{` ` ` `int` `arr[] = {2, 1, 3, 4, 2, 1, 5, 1, 7};` ` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]);` ` ` `cout << ` `"Maximum Difference = "` ` ` `<<maxDifference(arr, n);` ` ` `return` `0; ` `} ` |

## Java

`// Java implementation to find the maximum difference ` `// of first and last index of array elements` `import` `java.util.HashMap;` `import` `java.util.Map;` ` ` `public` `class` `MaxDiffIndexHashing {` ` ` ` ` `static` `class` `Element {` ` ` `int` `first;` ` ` `int` `second;` ` ` ` ` `public` `Element() {` ` ` `super` `();` ` ` `}` ` ` ` ` `public` `Element(` `int` `first, ` `int` `second) {` ` ` `super` `();` ` ` `this` `.first = first;` ` ` `this` `.second = second;` ` ` `}` ` ` `}` ` ` ` ` `public` `static` `void` `main(String[] args) {` ` ` ` ` `int` `arr[]={` `2` `, ` `1` `, ` `3` `, ` `4` `, ` `2` `, ` `1` `, ` `5` `, ` `1` `, ` `7` `};` ` ` `System.out.println(` `"Maximum Difference= "` `+ maxDiffIndices(arr));` ` ` `}` ` ` ` ` `private` `static` `int` `maxDiffIndices(` `int` `[] arr) {` ` ` `int` `n = arr.length;` ` ` `int` `maxDiffIndex = ` `0` `;` ` ` `Map<Integer, Element> map = ` `new` `HashMap<Integer, Element>();` ` ` ` ` `for` `(` `int` `i = ` `0` `; i < n; i++) {` ` ` `if` `(map.containsKey(arr[i])) {` ` ` `Element e = map.get(arr[i]);` ` ` `e.second = i;` ` ` `} ` `else` `{` ` ` `Element e = ` `new` `Element();` ` ` `e.first = i;` ` ` `map.put(arr[i], e);` ` ` `}` ` ` ` ` `}` ` ` ` ` `for` `(Map.Entry<Integer, Element> entry : map.entrySet()) {` ` ` `Element e = entry.getValue();` ` ` `if` `((e.second - e.first) > maxDiffIndex)` ` ` `maxDiffIndex = e.second - e.first;` ` ` `}` ` ` ` ` `return` `maxDiffIndex;` ` ` `}` ` ` `} ` |

## C#

`// C# implementation to find the maximum difference ` `// of first and last index of array elements ` `using` `System;` `using` `System.Collections.Generic;` ` ` `public` `class` `MaxDiffIndexHashing ` `{ ` ` ` ` ` `class` `Element { ` ` ` `public` `int` `first; ` ` ` `public` `int` `second; ` ` ` ` ` `public` `Element() { ` ` ` `} ` ` ` ` ` `public` `Element(` `int` `first, ` `int` `second) { ` ` ` `this` `.first = first; ` ` ` `this` `.second = second; ` ` ` `} ` ` ` `} ` ` ` ` ` `public` `static` `void` `Main(String[] args) { ` ` ` ` ` `int` `[]arr={2, 1, 3, 4, 2, 1, 5, 1, 7}; ` ` ` `Console.WriteLine(` `"Maximum Difference= "` `+ maxDiffIndices(arr)); ` ` ` `} ` ` ` ` ` `private` `static` `int` `maxDiffIndices(` `int` `[] arr) { ` ` ` `int` `n = arr.Length; ` ` ` `int` `maxDiffIndex = 0; ` ` ` `Dictionary<` `int` `, Element> map = ` `new` `Dictionary<` `int` `, Element>(); ` ` ` ` ` `for` `(` `int` `i = 0; i < n; i++) { ` ` ` `if` `(map.ContainsKey(arr[i])) { ` ` ` `Element e = map[arr[i]]; ` ` ` `e.second = i; ` ` ` `} ` `else` `{ ` ` ` `Element e = ` `new` `Element(); ` ` ` `e.first = i; ` ` ` `map.Add(arr[i], e); ` ` ` `} ` ` ` ` ` `} ` ` ` ` ` `foreach` `(KeyValuePair<` `int` `, Element> entry ` `in` `map) { ` ` ` `Element e = entry.Value; ` ` ` `if` `((e.second - e.first) > maxDiffIndex) ` ` ` `maxDiffIndex = e.second - e.first; ` ` ` `} ` ` ` ` ` `return` `maxDiffIndex; ` ` ` `} ` `} ` ` ` `// This code is contributed by 29AjayKumar` |

**Output:**

Maximum Difference = 6

Time Complexity: O(n)

This article is contributed by **Ayush Jauhari**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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