longest-contgous-subsequence.cpp

#include<bits/stdc++.h>
using namespace std;

// Returns length of the longest contiguous subsequence
int findLongestConseqSubseq(int arr[], int n)
{
    unordered_set<int> S;
    int ans = 0;

    // Hash all the array elements
    for (int i = 0; i < n; i++)
        S.insert(arr[i]);

    // check each possible sequence from the start
    // then update optimal length
    for (int i=0; i<n; i++)
    {
        // if current element is the starting
        // element of a sequence
        if (S.find(arr[i]-1) == S.end())
        {
            // Then check for next elements in the
            // sequence
            int j = arr[i];
            while (S.find(j) != S.end())
                j++;

            // update  optimal length if this length
            // is more
            ans = max(ans, j - arr[i]);
        }
    }
    return ans;
}

// Driver program
int main()
{
    int arr[] =  {1, 9, 3, 10, 4, 20, 2};
    int n = sizeof arr/ sizeof arr[0];
    cout << "Length of the Longest contiguous subsequence is "
         << findLongestConseqSubseq(arr, n);
    return 0;
}