You are given an integer array nums
and an integer k
.
Split the array into some number of non-empty subarrays. The cost of a split is the sum of the importance value of each subarray in the split.
Let trimmed(subarray)
be the version of the subarray where all numbers which appear only once are removed.
trimmed([3,1,2,4,3,4]) = [3,4,3,4].
The importance value of a subarray is k + trimmed(subarray).length
.
[1,2,3,3,3,4,4]
, then trimmed([1,2,3,3,3,4,4]) = [3,3,3,4,4].
The importance value of this subarray will be k + 5
.Return the minimum possible cost of a split of nums
.
A subarray is a contiguous non-empty sequence of elements within an array.
Example 1:
Input: nums = [1,2,1,2,1,3,3], k = 2 Output: 8 Explanation: We split nums to have two subarrays: [1,2], [1,2,1,3,3]. The importance value of [1,2] is 2 + (0) = 2. The importance value of [1,2,1,3,3] is 2 + (2 + 2) = 6. The cost of the split is 2 + 6 = 8. It can be shown that this is the minimum possible cost among all the possible splits.
Example 2:
Input: nums = [1,2,1,2,1], k = 2 Output: 6 Explanation: We split nums to have two subarrays: [1,2], [1,2,1]. The importance value of [1,2] is 2 + (0) = 2. The importance value of [1,2,1] is 2 + (2) = 4. The cost of the split is 2 + 4 = 6. It can be shown that this is the minimum possible cost among all the possible splits.
Example 3:
Input: nums = [1,2,1,2,1], k = 5 Output: 10 Explanation: We split nums to have one subarray: [1,2,1,2,1]. The importance value of [1,2,1,2,1] is 5 + (3 + 2) = 10. The cost of the split is 10. It can be shown that this is the minimum possible cost among all the possible splits.
Constraints:
1 <= nums.length <= 1000
0 <= nums[i] < nums.length
1 <= k <= 109
When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:
To find the cheapest way to split a sequence of numbers, the brute force approach is to consider every single possible way to cut it up. We then calculate the total cost for each complete split and simply pick the one with the lowest cost.
Here's how the algorithm would work step-by-step:
def min_cost_split_brute_force(numbers, k_value):
def get_trimmed_length(subarray):
counts = {}
for number in subarray:
counts[number] = counts.get(number, 0) + 1
trimmed_len = 0
for number in subarray:
if counts[number] > 1:
trimmed_len += 1
return trimmed_len
def get_subarray_cost(subarray):
return k_value + get_trimmed_length(subarray)
memoization_cache = {}
def find_minimum_cost_from_index(start_index):
if start_index >= len(numbers):
return 0
if start_index in memoization_cache:
return memoization_cache[start_index]
# Initialize minimum cost for the split starting at `start_index` to a very large number.
minimum_split_cost = float('inf')
# Iterate through all possible end points to define the first subarray of this recursive call.
for end_index in range(start_index, len(numbers)):
current_subarray = numbers[start_index : end_index + 1]
cost_of_this_subarray = get_subarray_cost(current_subarray)
# Recursively find the minimum cost for the rest of the array after the current split.
cost_of_rest_of_array = find_minimum_cost_from_index(end_index + 1)
total_cost_for_this_path = cost_of_this_subarray + cost_of_rest_of_array
minimum_split_cost = min(minimum_split_cost, total_cost_for_this_path)
# Store the result to avoid recomputing the minimum cost for this starting index.
memoization_cache[start_index] = minimum_split_cost
return minimum_split_cost
return find_minimum_cost_from_index(0)
To find the cheapest way to split the list, we can solve it piece by piece. We figure out the best cost for splitting smaller initial portions of the list and use those answers to efficiently calculate the best cost for larger portions, avoiding repetitive work.
Here's how the algorithm would work step-by-step:
from collections import Counter
def min_cost(nums, k):
array_length = len(nums)
# This DP array stores the minimum cost to split the subarray from the start up to index i-1.
min_cost_dp = [float('inf')] * (array_length + 1)
min_cost_dp[0] = 0
# Iterate through each possible end point of a split, building up the solution.
for end_index in range(1, array_length + 1):
current_min_total_cost = float('inf')
frequency_counter = Counter()
trimmed_length = 0
# Consider all possible start points for the final subarray ending at end_index-1.
for start_index in range(end_index - 1, -1, -1):
current_num = nums[start_index]
frequency_counter[current_num] += 1
# Calculate the trimmed_length for the current subarray [start_index...end_index-1].
if frequency_counter[current_num] == 2:
trimmed_length += 2
elif frequency_counter[current_num] > 2:
trimmed_length += 1
cost_of_last_subarray = k + trimmed_length
# Combine the cost of the final subarray with the optimal cost of the preceding part.
total_cost_for_this_split = min_cost_dp[start_index] + cost_of_last_subarray
current_min_total_cost = min(current_min_total_cost, total_cost_for_this_split)
min_cost_dp[end_index] = current_min_total_cost
return min_cost_dp[array_length]
Case | How to Handle |
---|---|
Empty input array `nums` | The cost should be 0 as there are no subarrays to form and no splits to make. |
Single element array `nums` | The only split is the array itself, so the cost is its subarray cost (1) with zero splits. |
The value of `k` is zero | The cost of splitting becomes free, so the optimal solution may involve many small subarrays. |
The value of `k` is very large | A large `k` heavily penalizes splits, making the optimal solution likely to be splitting the array into just one subarray. |
All elements in `nums` are unique | The trimmed length of any subarray is always its actual length, simplifying the subarray cost calculation to just `2 * length`. |
All elements in `nums` are identical | The trimmed length of any subarray of length `L > 1` is `L - (L-1) = 1`, making its cost `L + 1`. |
Maximum input size for `nums` (e.g., N=1000) | The solution must be more efficient than exponential, likely an O(N^2) dynamic programming approach to avoid a timeout. |
Input `nums` contains negative numbers or zeros | The values of the numbers do not affect the logic, only their frequencies, so the algorithm handles these cases correctly. |