Given an array of **n** integers. The problem is to find maximum length of the subsequence with difference between adjacent elements in the subsequence as either 0 or 1. Time Complexity of O(n) is required.**Examples:**

Input : arr[] = {2, 5, 6, 3, 7, 6, 5, 8} Output : 5 The subsequence is {5, 6, 7, 6, 5}. Input : arr[] = {-2, -1, 5, -1, 4, 0, 3} Output : 4 The subsequence is {-2, -1, -1, 0}.

**Method 1:** Previously an approach having time complexity of O(n^{2}) have been discussed in this post.**Method 2 (Efficient Approach):** The idea is to create a hash map having tuples in the form **(ele, len)**, where **len** denotes the length of the longest subsequence ending with the element **ele**. Now, for each element arr[i] we can find the length of the values arr[i]-1, arr[i] and arr[i]+1 in the hash table and consider the maximum among them. Let this maximum value be **max**. Now, the length of longest subsequence ending with arr[i] would be **max+1**. Update this length along with the element arr[i] in the hash table. Finally, the element having the maximum length in the hash table gives the maximum length subsequence.

## C++

`// C++ implementation to find maximum length` `// subsequence with difference between adjacent` `// elements as either 0 or 1` `#include <bits/stdc++.h>` `using` `namespace` `std;` ` ` `// function to find maximum length subsequence` `// with difference between adjacent elements as` `// either 0 or 1` `int` `maxLenSub(` `int` `arr[], ` `int` `n)` `{` ` ` `// hash table to map the array element with the` ` ` `// length of the longest subsequence of which` ` ` `// it is a part of and is the last element of` ` ` `// that subsequence` ` ` `unordered_map<` `int` `, ` `int` `> um;` ` ` ` ` `// to store the maximum length subsequence` ` ` `int` `maxLen = 0;` ` ` ` ` `// traverse the array elements` ` ` `for` `(` `int` `i=0; i<n; i++)` ` ` `{` ` ` `// initialize current length` ` ` `// for element arr[i] as 0` ` ` `int` `len = 0;` ` ` ` ` `// if 'arr[i]-1' is in 'um' and its length of` ` ` `// subsequence is greater than 'len'` ` ` `if` `(um.find(arr[i]-1) != um.end() && len < um[arr[i]-1])` ` ` `len = um[arr[i]-1];` ` ` ` ` `// if 'arr[i]' is in 'um' and its length of` ` ` `// subsequence is greater than 'len' ` ` ` `if` `(um.find(arr[i]) != um.end() && len < um[arr[i]])` ` ` `len = um[arr[i]];` ` ` ` ` `// if 'arr[i]+1' is in 'um' and its length of` ` ` `// subsequence is greater than 'len' ` ` ` `if` `(um.find(arr[i]+1) != um.end() && len < um[arr[i]+1])` ` ` `len = um[arr[i]+1]; ` ` ` ` ` `// update arr[i] subsequence length in 'um' ` ` ` `um[arr[i]] = len + 1;` ` ` ` ` `// update maximum length` ` ` `if` `(maxLen < um[arr[i]]) ` ` ` `maxLen = um[arr[i]];` ` ` `}` ` ` ` ` `// required maximum length subsequence` ` ` `return` `maxLen; ` `}` ` ` `// Driver program to test above` `int` `main()` `{` ` ` `int` `arr[] = {2, 5, 6, 3, 7, 6, 5, 8};` ` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]);` ` ` `cout << ` `"Maximum length subsequence = "` ` ` `<< maxLenSub(arr, n);` ` ` `return` `0;` `}` |

## Java

`// Java implementation to find maximum length` `// subsequence with difference between adjacent` `// elements as either 0 or 1` `import` `java.util.HashMap;` `class` `GFG` `{` ` ` ` ` `// function to find maximum length subsequence` ` ` `// with difference between adjacent elements as` ` ` `// either 0 or 1` ` ` `public` `static` `int` `maxLengthSub(` `int` `[] arr)` ` ` `{` ` ` ` ` `// to store the maximum length subsequence` ` ` `int` `max_val = ` `0` `;` ` ` `int` `start = ` `0` `;` ` ` ` ` `// hash table to map the array element with the` ` ` `// length of the longest subsequence of which` ` ` `// it is a part of and is the last element of` ` ` `// that subsequence` ` ` `HashMap<Integer, Integer> map = ` `new` `HashMap<>();` ` ` `// traverse the array elements` ` ` `for` `(` `int` `i = ` `0` `; i < arr.length; i++)` ` ` `{` ` ` ` ` `// initialize current length` ` ` `// for element arr[i] as 0` ` ` `int` `temp = ` `0` `;` ` ` `if` `(map.containsKey(arr[i] - ` `1` `))` ` ` `{` ` ` `temp = map.get(arr[i] - ` `1` `);` ` ` `}` ` ` `if` `(map.containsKey(arr[i]))` ` ` `{` ` ` `temp = Math.max(temp, map.get(arr[i]));` ` ` `}` ` ` ` ` `if` `(map.containsKey(arr[i] + ` `1` `))` ` ` `{` ` ` `temp = Math.max(temp, map.get(arr[i] + ` `1` `));` ` ` `}` ` ` `temp++;` ` ` ` ` `// update maximum length` ` ` `if` `(temp > max_val)` ` ` `{` ` ` `max_val = temp;` ` ` `}` ` ` `map.put(arr[i], temp);` ` ` `}` ` ` ` ` `// required maximum length subsequence` ` ` `return` `max_val;` ` ` `}` ` ` ` ` `// Driver Code` ` ` `public` `static` `void` `main(String[] args)` ` ` `{` ` ` `int` `arr[] = {` `2` `, ` `5` `, ` `6` `, ` `3` `, ` `7` `, ` `6` `, ` `5` `, ` `8` `};` ` ` `System.out.println(maxLengthSub(arr));` ` ` `}` `}` `// This code is contributed` `// by tushar jajodia` |

## Python3

`# Python3 implementation to find maximum` `# length subsequence with difference between` `# adjacent elements as either 0 or 1` `from` `collections ` `import` `defaultdict` `# Function to find maximum length subsequence with` `# difference between adjacent elements as either 0 or 1` `def` `maxLenSub(arr, n):` ` ` `# hash table to map the array element with the` ` ` `# length of the longest subsequence of which it is a` ` ` `# part of and is the last element of that subsequence` ` ` `um ` `=` `defaultdict(` `lambda` `:` `0` `)` ` ` ` ` `# to store the maximum length subsequence` ` ` `maxLen ` `=` `0` ` ` ` ` `# traverse the array elements` ` ` `for` `i ` `in` `range` `(` `0` `, n):` ` ` ` ` `# initialize current length` ` ` `# for element arr[i] as 0` ` ` `length ` `=` `0` ` ` ` ` `# if 'arr[i]-1' is in 'um' and its length of` ` ` `# subsequence is greater than 'len'` ` ` `if` `(arr[i]` `-` `1` `) ` `in` `um ` `and` `length < um[arr[i]` `-` `1` `]:` ` ` `length ` `=` `um[arr[i]` `-` `1` `]` ` ` ` ` `# if 'arr[i]' is in 'um' and its length of` ` ` `# subsequence is greater than 'len'` ` ` `if` `arr[i] ` `in` `um ` `and` `length < um[arr[i]]:` ` ` `length ` `=` `um[arr[i]]` ` ` ` ` `# if 'arr[i]+1' is in 'um' and its length of` ` ` `# subsequence is greater than 'len' ` ` ` `if` `(arr[i]` `+` `1` `) ` `in` `um ` `and` `length < um[arr[i]` `+` `1` `]:` ` ` `length ` `=` `um[arr[i]` `+` `1` `]` ` ` ` ` `# update arr[i] subsequence length in 'um'` ` ` `um[arr[i]] ` `=` `length ` `+` `1` ` ` ` ` `# update maximum length` ` ` `if` `maxLen < um[arr[i]]:` ` ` `maxLen ` `=` `um[arr[i]]` ` ` ` ` `# required maximum length subsequence` ` ` `return` `maxLen` `# Driver program to test above` `if` `__name__ ` `=` `=` `"__main__"` `:` ` ` `arr ` `=` `[` `2` `, ` `5` `, ` `6` `, ` `3` `, ` `7` `, ` `6` `, ` `5` `, ` `8` `]` ` ` `n ` `=` `len` `(arr)` ` ` `print` `(` `"Maximum length subsequence ="` `, maxLenSub(arr, n))` ` ` `# This code is contributed by Rituraj Jain` |

## Javascript

`<script>` `// Javascript implementation to find maximum length` `// subsequence with difference between adjacent` `// elements as either 0 or 1` ` ` `// function to find maximum length subsequence` `// with difference between adjacent elements as` `// either 0 or 1` `function` `maxLenSub(arr, n)` `{` ` ` `// hash table to map the array element with the` ` ` `// length of the longest subsequence of which` ` ` `// it is a part of and is the last element of` ` ` `// that subsequence` ` ` `var` `um = ` `new` `Map();` ` ` ` ` `// to store the maximum length subsequence` ` ` `var` `maxLen = 0;` ` ` ` ` `// traverse the array elements` ` ` `for` `(` `var` `i=0; i<n; i++)` ` ` `{` ` ` `// initialize current length` ` ` `// for element arr[i] as 0` ` ` `var` `len = 0;` ` ` ` ` `// if 'arr[i]-1' is in 'um' and its length of` ` ` `// subsequence is greater than 'len'` ` ` `if` `(um.has(arr[i]-1) && len < um.get(arr[i]-1))` ` ` `len = um.get(arr[i]-1);` ` ` ` ` `// if 'arr[i]' is in 'um' and its length of` ` ` `// subsequence is greater than 'len' ` ` ` `if` `(um.has(arr[i]) && len < um.get(arr[i]))` ` ` `len = um.get(arr[i]);` ` ` ` ` `// if 'arr[i]+1' is in 'um' and its length of` ` ` `// subsequence is greater than 'len' ` ` ` `if` `(um.has(arr[i]+1) && len < um.get(arr[i]+1))` ` ` `len = um.get(arr[i]+1); ` ` ` ` ` `// update arr[i] subsequence length in 'um' ` ` ` `um.set(arr[i], len + 1);` ` ` ` ` `// update maximum length` ` ` `if` `(maxLen < um.get(arr[i])) ` ` ` `maxLen = um.get(arr[i]);` ` ` `}` ` ` ` ` `// required maximum length subsequence` ` ` `return` `maxLen; ` `}` ` ` `// Driver program to test above` `var` `arr = [2, 5, 6, 3, 7, 6, 5, 8];` `var` `n = arr.length;` `document.write( ` `"Maximum length subsequence = "` ` ` `+ maxLenSub(arr, n));` `// This code is contributed by itsok.` `</script>` |

**Output:**

Maximum length subsequence = 5

**Time Complexity:** O(n) **Auxiliary Space:** O(n)

Thanks to **Neeraj** for suggesting the above solution in the comments of this post.

This article is contributed by **Ayush Jauhari**. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.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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