Find all unique pairs of maximum and second maximum elements over all sub-arrays in O(NlogN)

Let (a, b) represent the ordered pair of the second maximum and the maximum element of an array respectively. We need to find all such unique pairs overall contiguous sub-arrays of a given array.

Examples:

Input: Arr = [ 1, 2, 3, 4, 5 ]
Output: (1, 2) (2, 3) (3, 4) (4, 5)

Input: Arr = [ 1, 1, 2 ]
Output: (1, 1) (1, 2)

Input: Arr = [ 1, 2, 6, 4, 5 ]
Output: (1, 2) (2, 6) (4, 5) (4, 6) (5, 6)

Brute Force Approach:

  • A simple way to solve this problem would be to scan each sub-array and find the maximum and second maximum element in that sub-array
  • This can be done in O(N^2) time
  • Then we can insert each pair in a set to ensure duplicates are removed, and then print them
  • Each insertion operation costs O(log(N)), pushing the final complexity to O(N^2log(N))

Efficient Approach:

  • It could bring down the complexity of finding pairs to O(N) by observing that an element X can form pairs with elements only till the closest element to the right which is greater than X.
  • To see why this holds, consider X = 4 in the next example.
     Arr = {1, 4, 5, 3, 2, 1}
    • It could see that 5 > 4 is the nearest element to the right which is greater than 4. (4, 5) forms a pair considering the sub-array [4, 5].
    • Other sub-arrays, that start with 4 must include 5. Considering one of them, if another element Y >=5 exists in the sub-array, then (5, Y) will be the pair for that sub-array.
    • Else either (4, 5) will be formed or (Z, 5) will be formed, where Z is the max element to the right of 5 in the sub-array.
    • In any cases, 4 cannot form a pair with any element to the right of 5.
  • Using this observation, we can implement the logic using stack which brings down the pair generation complexity to O(N).
  • Each pair can be inserted into a set for eliminating duplicates, giving a final time complexity of O(Nlog(N))

Below is the implementation of the above approach

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// C++ implementation
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the set of pairs
set<pair<int, int>> 
     pairs(vector<int>& arr)
{
    stack<int> st;
    set<pair<int, int>> pairs;
  
    // Push first element into stack
    st.push(arr[0]);
  
    // For each element 'X' in arr,
    // pop the stack while top Element
    // is smaller than 'X' and form a pair.
    // If the stack is not empty after
    // the previous operation, create
    // a pair. Push X into the stack.
  
    for (int i = 1; i < arr.size(); ++i) {
        while (!st.empty() &&
                arr[i] > st.top()) {
            pairs.insert(make_pair(st.top(),
                                    arr[i]));
            st.pop();
        }
        if (!st.empty()) {
            pairs.insert(make_pair(min(st.top(),
                                       arr[i]),
                                   max(st.top(),
                                      arr[i])));
        }
        st.push(arr[i]);
    }
    return pairs;
}
  
int main()
{
    vector<int> vec = { 1, 2, 6, 4, 5 };
  
    set<pair<int, int> > st = pairs(vec);
    cout << "Total Number of valid pairs is :" 
                   << (int)st.size() << "\n";
    for (auto& x : st) {
        cout << "(" << x.first << ", "
                       << x.second << ") ";
    }
    return 0;
}

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Output:

Total Number of valid pairs is :5
(1, 2) (2, 6) (4, 5) (4, 6) (5, 6)


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