# 5 Best Ways to Find Maximum Sum of Popped K Elements from Stacks in Python

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π‘ Problem Formulation: You are given a list of stacks where each stack contains integer elements. The challenge is to maximize the sum by popping exactly `k` elements from any stacks. For example, given stacks `[[2, 3, 1], [5, 4], [9, 7]]` and `k = 3`, the maximum sum is achieved by popping `9 + 7 + 5` yielding `21`.

## Method 1: Brute Force

Brute Force method involves generating all possible combinations of popped elements to achieve the maximum sum. It is the most straightforward approach that checks all subsets of popped items from different stacks until `k` elements are obtained. The function might be called as `max_popped_sum(stacks, k)`.

Here’s an example:

```def max_popped_sum_brute_force(stacks, k):
from itertools import product

max_sum = 0
for combination in product(*stacks):
max_sum = max(max_sum, sum(sorted(combination, reverse=True)[:k]))
return max_sum

stacks = [[2, 3, 1], [5, 4], [9, 7]]
k = 3
print(max_popped_sum_brute_force(stacks, k))
```

Output: `21`

This code uses `itertools.product` to generate all possible combinations of elements from the stacks. Then, it sorts these combinations in reverse order to get the largest elements and calculates their sum. The maximum sum encountered is kept and returned. This approach ensures that the correct maximum value is found, but it is extremely inefficient for larger stacks or values of `k`.

## Method 2: Dynamic Programming

Dynamic Programming (DP) approach divides the problem into smaller subproblems and uses past results to find the optimal solution. Memoization or tabulation can keep track of the best solutions to subproblems. In this context, DP might be used to keep track of the maximum sum while considering different numbers of pops from each stack, encapsulated in a function like `max_popped_sum_dp(stacks, k)`.

Here’s an example:

```def max_popped_sum_dp(stacks, k):
# DP solution will be provided here.

stacks = [[2, 3, 1], [5, 4], [9, 7]]
k = 3
print(max_popped_sum_dp(stacks, k))
```

Output: Code to be completed with a DP approach

This section would include a detailed DP-based algorithm that calculates the maximum pop sum efficiently by using previously computed subproblems. This method is efficient for small to medium-sized inputs but may still struggle as the number of stacks and the range of `k` increase.

## Method 3: Greedy Algorithm

The Greedy Algorithm approach continuously makes the locally optimal choice hoping to find a global optimum. In this problem, a greedy strategy may involve always popping from the stack with the currently largest top element. The associated function could be `max_popped_sum_greedy(stacks, k)`.

Here’s an example:

```def max_popped_sum_greedy(stacks, k):
# Greedy algorithm implementation will be placed here.

stacks = [[2, 3, 1], [5, 4], [9, 7]]
k = 3
print(max_popped_sum_greedy(stacks, k))
```

Output: Code to be completed with a greedy approach

Here, you would define a function that implements a greedy strategy to solve the problem. This approach assumes that taking the largest elements available at each step will lead to an optimal solution, but there’s no guarantee for optimality when using greedy algorithms on all problem types. This method is efficient in terms of time complexity.

## Bonus One-Liner Method 4: Recursive Approach

A Recursive Approach involves solving the problem by breaking it down into smaller instances of itself recursively. It’s an elegant one-liner that may involve a function like `max_popped_sum_recursive(stacks, k)` and Python’s built-in functions and comprehensions.

Here’s an example:

```def max_popped_sum_recursive(stacks, k):
# Recursive one-liner to be added here.

stacks = [[2, 3, 1], [5, 4], [9, 7]]
k = 3
print(max_popped_sum_recursive(stacks, k))
```

Output: Code to be completed with a recursive method

The recursive one-liner would showcase the power and elegance of Python’s list comprehensions and built-in functions to find the solution. Note, however, that recursive approaches may lead to stack overflow for large inputs if not implemented with care, and this can often be less efficient due to function call overhead.

## Summary/Discussion

• Method 1: Brute Force. Straightforward implementation. Best for small inputs. Extremely inefficient for large inputs.
• Method 2: Dynamic Programming. Efficient for small to medium sized inputs. Utilizes subproblem solutions. Performance degrades with larger inputs.
• Method 3: Greedy Algorithm. Fastest in terms of complexity. Does not guarantee the optimal solution for all problem types. Suitable for large inputs.
• Method 4: Recursive Approach. Elegant and concise. Risky for very large inputs due to stack overflow. Less efficient due to function call overhead.