Reduce the array to a single element by repeatedly removing an element from any increasing pair

Reducing an array to a single element by repeatedly removing element is done by the following criteria −

Select indices i and j such that i < j and arr[i] < arr[j] and convert one of the two elements to 0.

Problem Statement

Given an array arr[] containing positive integers. Find if the array can be reduced to a single element by repeatedly removing an element from any increasing pair. If possible return true along with the indices chosen and the index of the element that is removed.

Sample Example 1

Input

arr[] = {5, 7, 10, 2, 4, 8}

Output

True
0 1 1
0 2 2
4 5 4
3 5 3
0 5 5

Explanation

• Step 1 − Choose elements at index 0 and 1 i.e. 5 and 7. Then removing 7 i.e. element at 1st index.

• Step 2 − Choose elements at index 0 and 2 i.e. 5 and 10. Then removing 10 i.e. element at 2nd index.

• Step 3 − Choose elements at index 4 and 5 i.e. 4 and 8. Then removing 4 i.e. element at 4th index.

• Step 4 − Choose elements at index 3 and 5 i.e. 2 and 8. Then removing 2 i.e. element at 3rd index.

• Step 5 − Choose elements at index 0 and 5 i.e. 5 and 8. Then removing 8 i.e. element at 5th index.

Sample Example 2

Input

arr[] = {9, 3, 5}

Output

False

Explanation

• Step 1 − Choose elements at index 1 and 2 i.e. 3 and 5. Then removing 5 i.e. element at 2nd index.

• Step 2 − Choose elements at index 0 and 1 i.e. 9 and 3. Since 3 is not greater than 9, so the array cannot be reduced.

Solution Approach

The first step to the solution to this problem is to choose a valid set of indices first. Then thenest steps include deciding onto which element to delete. If we choose element of 0th index in the pair then remove the non-zero index element else remove the smaller element. Repeat the steps until a single element is left. If a single element is left, return true else return false.

Pseudocode −

procedure order(a: array of integer, n: integer)
   g <- empty queue of integers
   first <- a[0]
   p <- 0
   for i <- 1 to n - 1
      if a[i] > first
         g.push(i)
   while g.size() > 0 do
      index <- g.front()
      g.pop()
      remove <- index
      while remove > p do
         print remove, " ", index, " ", remove
         remove <- remove - 1
      end while
      print "0 ", index, " ", index
      p <- index
   end while
end procedure
procedure canReduced(arr: array of integer, N: integer)
   if arr[0] > arr[N - 1] then
      print "False"
   else
      print "True"
      order(arr, N)
   end if
end procedure

Example: C++ Implementation

The following code uses the above approach to solve the problem.

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

// Function to print the order of indices of converted numbers
void order(int a[], int n){
   // Values of indices with numbers > first index
   queue<int> g;
   int first = a[0];
   // Index of the closest consecutive number to index 0
   int p = 0;
   // Pushing the indices
   for (int i = 1; i < n; i++){
      if (a[i] > first)
         g.push(i);
   }
   // Traverse the queue
   while (g.size() > 0){
      // Index of the closest number > arr[0]
      int index = g.front();
      g.pop();
      int remove = index;
      // Remove elements present in indices [1, remove - 1]
      while (--remove > p) {
         cout << remove << " "
            << index << " "
            << remove << endl;
      }
      cout << 0 << " " << index << " "
      << index << endl;
      // Updating the previous index to index
      p = index;
   }
}
// Function to check if array arr[] can be reduced to single element or not
void canReduced(int arr[], int N){
   // Element can't be reduced to single element
   if (arr[0] > arr[N - 1]){
      cout << "False" << endl;
   }
   else {
      cout << "True" << endl;
      order(arr, N);
   }
}
int main(){
   // Given array arr[]
   int arr[] = {5, 7, 10, 2, 4, 8};
   int N = sizeof(arr) / sizeof(arr[0]);
   canReduced(arr, N);
   return 0;
}

Output

True
0 1 1
0 2 2
4 5 4
3 5 3
0 5 5

Conclusion

In conclusion, the given code provides solution to the problem using a queue data structure. The tiem complexity is O(N) where N is the length of the input array as we iterate over the array once to find the increasing pairs and performs an operation on each pair taking constant time. The space complexity is also O(N) due to the queue used to store the indices. The order of removing the elements can alter the final result. The above solution removes elements in decreasing order indices in na increasing pair.