2155. All Divisions With the Highest Score of a Binary Array

Problem Description

You are given a binary array, nums, that contains only 0s and 1s. The task is to find all indices at which you can divide this array into two subarrays, nums_left and nums_right, such that the division score is maximized. The division score is defined as the count of 0s in nums_left plus the count of 1s in nums_right.

The array can be divided at any index i, which creates nums_left with elements from the start of the array up to index i-1, and nums_right with elements from index i to the end. Note that if i is 0, then nums_left is empty and nums_right contains all the elements, while if i is the length of the array (n), then nums_left contains all the elements and nums_right is empty.

The goal is to return indices that yield the highest division score. The indices can be returned in any order.

Intuition

To solve this problem, we need to understand that at each potential division index, the score is affected by the number of zeros to the left and the number of ones to the right of that index. Therefore, if we know the total number of ones in the array beforehand, we can easily calculate the division score for each index without having to count the number of ones to the right every time.

Here's the approach to solve the problem:

1. Initialize two counters, left and right. left will hold the number of 0s that we've encountered so far as we iterate from the beginning of the array towards the end, and right starts off as the total count of 1s in the entire array.

2. Initialize a variable mx that will hold the maximum division score that we've found so far. Initially, this will be equal to right because before any division, all 1s would be in nums_right and this is the highest possible score at the start.

3. Start iterating through the array. With each element, update left and right accordingly. If you encounter a 0, increment left. If you encounter a 1, decrement right. This is because a zero would now be contributing to the division score on the left, and a one would no longer contribute to the score on the right after passing the division point.

4. At each index, calculate the current division score, t, as the sum of left and right. If it's equal to mx, append the current index + 1 to the list of indices with maximum score, ans. If the current score is greater than the max score found so far (mx), update mx with t and reset ans to a new list containing just the current index + 1. This keeps track of all indices where we have encountered the maximum score.

5. After we have finished iterating through the array, we will have a list of all indices at which the maximum division score can be achieved. We return this list.

By keeping track of the running count of zeros and ones, we optimize the solution by only iterating through the array once (O(n) time complexity), rather than recalculating the division score from scratch at each possible division point.

Solution Approach

The implementation follows a simple but efficient algorithm that ensures we only need a single pass through the input array, which is a key characteristic of an O(n) time complexity solution. The approach heavily relies on the usage of two variables to represent the current number of zeros and ones relevant to the potential division at any index in the array. Here's the detailed implementation approach:

1. Initialize a counter left to 0. This will track the number of 0s to the left of the current index as we iterate through the array.
2. Initialize a counter right to the sum of the elements in the nums array (since the array contains only 0s and 1s, this sum equals the number of 1s in the array).
3. Set the maximum score mx initially equal to right because, at the start (when the division point is at index 0 and nums_left is empty), all 1s are in nums_right.
4. Create an array ans with a single element 0 because initially, the highest score can be achieved when the division point is at index 0.
5. Iterate over the elements in the nums array using a loop. For each element at index i, do the following:
   • If the current element num is 0, increment left because we've found another 0 that contributes to the score when it's in nums_left.
   • If the current element is 1, decrement right as this 1 no longer contributes to the score in nums_right after moving past index i.
   • Calculate the temporary score t as the sum of left and right, which represents the current division score at index i.
   • Compare the temporary score t with the maximum score mx:
     • If t is equal to mx, append i + 1 to ans. The +1 is necessary because the division occurs after the current index, so the next index is the potential division point.
     • If t is greater than mx, update mx to t and set ans to a new list containing only i + 1, since we have found a new highest score and the previous indices are no longer valid.
6. Continue this process until the end of the array. After the loop ends, the ans array contains all indices where the maximum division score is achieved.
7. Finally, return the ans array.

The use of a single loop and constant-time update operations ensures that the overall time complexity remains linear, which is crucial for handling large arrays efficiently. The space complexity is also kept low since we are only using a handful of variables and an output array, avoiding any additional data structures that could increase the space usage.

Example Walkthrough

Let's walk through an example to illustrate the solution approach with a binary array nums.

Suppose nums = [0, 1, 0, 1, 1]. We want to find all indices where we can divide this array to maximize the division score.

Following the solution approach:

1. Initialize left to 0 and right to 3 since there are three 1s in the array.
2. Set mx to 3 because at the index 0, nums_right would contain all the 1s, giving the maximum possible score initially.
3. Create an array ans with a single element 0.
4. Start iterating over the array nums.

Let's iterate through each element and apply the steps:

• At index 0, nums[0] is 0. Increment left to 1 (now left = 1, right = 3), the score here is 1 + 3 = 4. Since 4 is greater than mx, we update mx to 4 and set ans to [1] (next index after division).
• At index 1, nums[1] is 1. Decrement right to 2 (now left = 1, right = 2), the score now is 1 + 2 = 3. The score 3 is not greater than mx (4), so we continue without updating mx or ans.
• At index 2, nums[2] is 0. Increment left to 2 (now left = 2, right = 2), the new score is 2 + 2 = 4. This equals the mx, thus we append 3 to ans (becoming [1, 3]).
• At index 3, nums[3] is 1. Decrement right to 1 (now left = 2, right = 1), and the score here is 2 + 1 = 3. No change to mx or ans since the score is less.
• At index 4, nums[4] is 1. Decrement right to 0 (now left = 2, right = 0), and the score is 2 + 0 = 2, which is again less than mx.

The iteration is complete, and the ans array [1, 3] contains all the indices that give the maximum division score of 4. Thus, dividing the array at indices 1 or 3 gives us both subarrays that maximize the division score.

Solution Implementation

Time and Space Complexity

The time complexity of the code is O(n), where n is the length of the input list nums. This is because the code iterates once over all the elements of the list, performing a constant amount of work for each element (incrementing left if it's a 0, decrementing right if it's a 1, and comparing and possibly updating the maximum score).

The space complexity of the code is O(k), where k is the number of indices at which the maximum score is achieved. In the worst-case scenario, every index could be a part of the answer, in which case the space complexity would be O(n). However, under normal circumstances where not every index is a solution, k is typically much less than n. Fundamental to the space complexity is the list ans, which stores the indices. Besides that, the algorithm uses only a constant amount of additional space for variables like left, right, mx, and t.

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