Longest subsequence with first and last element greater than all other elements

Given a permutation from 1 to N of size N. Find the length of the longest subsequence having the property that the first and last element is greater than all the other subsequence elements.

Examples:

Input : N = 6
4 2 6 5 3 1
Output : 3
The subsequence that has the longest size is [4, 2, 3] or [4, 2, 6] or [4, 2, 5]. No other subsequence holds the above property with first and last element greater than all remaining subsequence elements.



Input : N = 5
5 4 1 2 3
Output : 4
The longest subsequence is [5, 1, 2, 3] or [4, 1, 2, 3]. With 5 and 3 greater than all other subsequence elements i.e 1 and 2.

Approach:
Idea is to use Fenwick tree data structure.Start iterating from the highest value in the permutation to lowest one. For each iteration, move left or a right pointer to the current element only if the left pointer can be moved to more left and right pointer to more right.
If left index is L and right index is R then there are elements from [L+1, R-1] in between. For this particular iteration i, count the number of elements that are lesser than min(arr[L], arr[R]) using the fenwick tree.
Store each query of type {left+1, right-1, val, index}, where left and right are current left and right pointers and val is the min of arr[L] and arr[R] and idx is the index of that particular query.

  1. Sort the array in ascending order.
  2. Sort the queries according to val i.e min(arr[L], arr[R] ) in ascending order, initialize Fenwick array as 0.
  3. Start from the first query and traverse the array until the value in the array is less than equal to val. For each such element update the Fenwick tree with value equal to 1.
  4. Query the Fenwick array in the range l to r.
  5. Print maximum of all the query results .

Below is the implementation of the above approach:

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// C++ implementation of the above approach
  
#include <bits/stdc++.h>
using namespace std;
  
struct Query {
  
    int l, r, x, index;
};
  
struct Arrays {
      
    int val, index;
};
  
// Comparison functions
bool cmp1(Query q1, Query q2)
{
    return q1.x < q2.x;
}
  
bool cmp2(Arrays x, Arrays y)
{
    return x.val < y.val;
}
  
// Function to update the value in Fenwick tree
void update(int* Fenwick, int index, int val, int n)
{
    while (index <= n) {
          
        Fenwick[index] += val;
        index += index & (-index);
    }
}
  
// Function to return the query result
int query(int* Fenwick, int index, int n)
{
    int sum = 0;
      
    while (index > 0) {
          
        sum = sum + Fenwick[index];
        index -= index & (-index);
    }
      
    return sum;
}
  
// Function to return the length of subsequence
int maxLength(int n, vector<int>& v)
{
    int where[n + 2];
      
    memset(where, 0, sizeof where);
      
    Arrays arr[n];
  
    // Store the value in struct Array
    for (int i = 1; i <= n; ++i) {
          
        v[i - 1] = v[i - 1] - 1;
          
        int x = v[i - 1];
          
        where[x] = i - 1;
        arr[i - 1].val = x;
        arr[i - 1].index = i - 1;
    }
  
    // If less than 2 elements are 
    // present return that element.
    if (n <= 2) {
          
        cout << n << endl;
          
        return 0;
    }
  
    // Set the left and right pointers to extreme
    int left = n, right = 0, mx = 0;
      
    Query queries[4 * n];
      
    int j = 0;
    for (int i = n - 1; i >= 0; --i) {
  
        // Calculate left and right pointer index.
        left = min(left, where[i]);
        right = max(right, where[i]);
          
        int diff = right - left;
          
        if (diff == 0 || diff == 1) {
            continue;
        }
          
        int val1 = v[left];
        int val2 = v[right];
        int minn = min(val1, val2);
  
        // Store the queries from [L+1, R-1].
        queries[j].l = left + 1;
        queries[j].r = right - 1;
        queries[j].x = minn;
        queries[j].index = j;
          
        ++j;
    }
  
    int Fenwick[n + 1];
      
    memset(Fenwick, 0, sizeof Fenwick);
      
    int q = j - 1;
  
    // Sort array and queries for fenwick updates
    sort(arr, arr + n + 1, cmp2);
    sort(queries, queries + q + 1, cmp1);
      
    int curr = 0;
    int ans[q];
    memset(ans, 0, sizeof ans);
  
    // For each query calculate maxx for 
    // the answer and store it in ans array.
    for (int i = 0; i <= q; ++i) {
  
        while (arr[curr].val <= queries[i].x and curr < n) {
              
            update(Fenwick, arr[curr].index + 1, 1, n);
            curr++;
        }
  
        ans[queries[i].index] = 
                query(Fenwick, queries[i].r + 1, n) 
                - query(Fenwick, queries[i].l, n);
    }
      
    for (int i = 0; i <= q; ++i) {
        mx = max(mx, ans[i]);
    }
      
    // Mx will be mx + 2 as while calculating
    // mx, we excluded element 
    // at index left and right
    mx = mx + 2;
    return mx;
}
  
// Driver Code
int main()
{
    int n = 6;
    vector<int> v = { 4, 2, 6, 5, 3, 1 };
  
    cout << maxLength(n, v) << endl;
      
    return 0;
}

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Output:

3


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