Longest Increasing consecutive subsequence

Given N elements, write a program that prints the length of the longest increasing subsequence whose adjacent element difference is one.

Examples:

Input : a[] = {3, 10, 3, 11, 4, 5, 6, 7, 8, 12}
Output : 6
Explanation: 3, 4, 5, 6, 7, 8 is the longest increasing subsequence whose adjacent element differs by one.

Input : a[] = {6, 7, 8, 3, 4, 5, 9, 10}
Output : 5
Explanation: 6, 7, 8, 9, 10 is the longest increasing subsequence



Naive Approach: A normal approach will be to iterate for every element and find out the longest increasing subsequence. For any particular element, find the length of the subsequence starting from that element. Print the longest length of the subsequence thus formed. The time complexity of this approach will be O(n2).

Dynamic Programming Approach: Let DP[i] store the length of the longest subsequence which ends with A[i]. For every A[i], if A[i]-1 is present in the array before i-th index, then A[i] will add to the increasing subsequence which has A[i]-1. Hence DP[i] = DP[ index(A[i]-1) ] + 1. If A[i]-1 is not present in the array before i-th index, then DP[i]=1 since the A[i] element forms a subsequence which starts with A[i]. Hence the relation for DP[i] is:

If A[i]-1 is present before i-th index:

  • DP[i] = DP[ index(A[i]-1) ] + 1
  • else:

  • DP[i] = 1
  • Given below is the illustration of the above approach:

    // CPP program to find length of the
    // longest increasing subsequence
    // whose adjacent element differ by 1
    #include <bits/stdc++.h>
    using namespace std;
      
    // function that returns the length of the
    // longest increasing subsequence
    // whose adjacent element differ by 1
    int longestSubsequence(int a[], int n)
    {
        // stores the index of elements
        unordered_map<int, int> mp;
      
        // stores the length of the longest 
        // subsequence that ends with a[i]
        int dp[n];
        memset(dp, 0, sizeof(dp)); 
      
        int maximum = INT_MIN;
      
        // iterate for all element
        for (int i = 0; i < n; i++) {
      
            // if a[i]-1 is present before i-th index
            if (mp.find(a[i] - 1) != mp.end()) {
      
                // last index of a[i]-1
                int lastIndex = mp[a[i] - 1] - 1;
      
                // relation
                dp[i] = 1 + dp[lastIndex];
            }
            else
                dp[i] = 1;
      
            // stores the index as 1-index as we need to
            // check for occurrence, hence 0-th index
            // will not be possible to check
            mp[a[i]] = i + 1;
      
            // stores the longest length
            maximum = max(maximum, dp[i]);
        }
      
        return maximum;
    }
      
    // Driver Code
    int main()
    {
        int a[] = { 3, 10, 3, 11, 4, 5, 6, 7, 8, 12 };
        int n = sizeof(a) / sizeof(a[0]);
        cout << longestSubsequence(a, n);
        return 0;
    }

    Output:

    6
    

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



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