2790. Maximum Number of Groups With Increasing Length

Problem Description

In this problem, you are provided with an array called usageLimits which is 0-indexed and has a length of n. This array represents the maximum times each number, from 0 to n-1, can be used to create groups. The goal is to form the maximum number of groups under certain conditions:

• Each group is made up of distinct numbers, meaning a particular number can’t be included more than once within the same group.
• Each group must have more members than the previous one, except for the very first group.

You are required to determine the maximum number of groups that can be created while adhering to the above constraints.

Intuition

To tackle this problem, we need a strategy that systematically utilizes the availability constraints specified by usageLimits. Since we need each subsequent group to have more elements than the previous, we could benefit from starting with the least usage limits.

Hence, the first step in our approach is to sort the usageLimits array. This way, we start forming groups with numbers which have the lowest usage limits, thereby preserving the more usable numbers for later, potentially larger, groups.

Now, we have to keep track of how many groups we can form. To do this, we iterate through the sorted usageLimits array and aggregate a sum (s) of the elements, which helps us to understand the total available "slots" for forming groups at each step. The number of groups (k) is initially set to 0.

As we iterate, if the total available slots (sum s) are greater than the number of groups formed so far (k), this indicates that there is enough capacity to form a new group with one more member than the last. So we increment our group count (k) and then adjust our available slots by subtracting the size of the new group (k) from our sum (s).

The iteration continues until we've processed all elements in usageLimits. The final value of k will be the maximum number of groups we can form.

This approach leverages greedy and sorting techniques to efficiently satisfy the conditions of distinct numbers and strictly increasing group sizes.

Learn more about Greedy, Math, Binary Search and Sorting patterns.

Solution Approach

The implementation of the solution uses a sorting technique, iteration, and simple arithmetic calculations:

1. Sorting: We start by sorting the usageLimits array. Sorting is crucial because it ensures that we use the numbers with smaller limits first, allowing us to potentially create more groups.

2. Iteration: After sorting, we iterate over each element in the usageLimits array. This step helps us to accumulate the total number of times we can use the numbers to form groups.

3. Group Formation and Incrementation: We use two variables, k for the number of groups formed, and s for the aggregated sum of usageLimits. With each iteration, we check if the current sum s is greater than the number of groups k. If so, we can form a new group and we increment k. This check is essentially asking, "Do we have enough available numbers to form a group larger than the current largest group?"

4. Sum Adjustment: Once we increment k to add a new group, we must also adjust the sum s to reflect the creation of this group. This is achieved by subtracting k from s. The subtraction of k from s signifies that we've allocated k distinct numbers for the new largest group, and these numbers cannot be re-used for another group.

The strategy employs a greedy approach, where we "greedily" form new groups as soon as we have enough capacity to do so, and always ensure that the new group complies with the conditions (distinct numbers and strictly greater size than the previous group). The final k value when we exit the loop is the maximum number of groups we can form.

Here, we don’t need any complex data structures. Array manipulation, following the sorting, and the use of a couple of integer variables to keep track of the sum and group count suffice in implementing the solution.

In conclusion, the combination of sorting, greedy algorithms with a simple condition check and variable adjustment within a single scan of the array yield the solution for the maximum number of groups that can be formed under the given constraints.

Example Walkthrough

Let's illustrate the solution approach with a small example. Suppose we have the following usageLimits array:

1usageLimits = [3, 1, 2, 0]

Here’s how we might walk through the solution:

1. Sorting: First, we sort the usageLimits array in non-decreasing order:

   1usageLimits = [0, 1, 2, 3]

2. Iteration: Next, we begin to iterate over the sorted usageLimits array while keeping track of the sum s of the elements, and the number of formed groups k. Initially, s = 0 and k = 0.

3. Group Formation and Incrementation:

   • At the first iteration, s = 0 + 0 = 0 and k = 0. We cannot form a group yet because there are not enough distinct numbers.
   • At the second iteration, s = 0 + 1 = 1 and k = 0. We now have enough numbers to form a group with 1 distinct number, so k is incremented to 1.
   • At the third iteration, s = 1 + 2 = 3 and k = 1. There are enough numbers to form another group with 2 distinct numbers (k + 1), so k is incremented to 2.

4. Sum Adjustment: After each group formation, we need to adjust s for the new group size. After the third iteration, we subtract k from s:

   1s = 3 - 2 = 1

5. Continuing Iteration:

   • At the fourth and final iteration, s = 1 + 3 = 4 and k = 2. Here, we can form one last group with 3 distinct numbers since s (4) is greater than k (2), so k is incremented to 3.
   • We adjust s one last time:

   1s = 4 - 3 = 1

6. Result: Since there are no more elements in usageLimits, our iteration ends. The final value of k, which is 3, is the maximum number of groups that can be formed given the constraints. Each group having 1, 2, and 3 distinct members respectively.

The example demonstrates how by sorting the array and using a greedy approach, systematically checking and updating counts, we maximize the number of distinct groups that can be created following the rules specified.

Solution Implementation

Time and Space Complexity

The given Python code is a function that aims to determine the maximum number of increasing groups from a list of usage limits. Here's the analysis of its time and space complexity:

Time Complexity

The most expensive operation in the algorithm is the sort operation, which has a time complexity of O(n log n), where n is the number of elements in usageLimits. After sorting, the function then iterates over the sorted list once, producing an additional O(n) complexity. Therefore, the total time complexity of the algorithm is dominated by the sorting step, resulting in O(n log n) overall.

Space Complexity

As for space complexity, the sorting operation can be O(n) in the worst case or O(log n) if an in-place sort like Timsort (used by Python's .sort() method) is applied efficiently. The rest of the algorithm uses a fixed amount of additional variables (k and s) and does not rely on any additional data structures that depend on the input size. Thus, the space complexity is O(1) for the additional variables used, but due to the sort's potential additional space, it could be O(n) or O(log n). Since Python's sort is typically implemented in a way that aims to minimize space overhead, the expected space complexity in practical use cases would likely be closer to O(log n).

Therefore, the final space complexity of the algorithm is O(log n) under typical conditions.

Note: The space complexity stated above assumes typical conditions based on Python's sorting implementation. The actual space complexity could vary depending on the specific details of the sort implementation in the environment where the code is executed.

Learn more about how to find time and space complexity quickly using problem constraints.