Put Marbles in Bags

Updated on 15 July, 2025
Put Marbles in Bags header image

Problem Statement

In this problem, you are provided with k bags and a zero-indexed integer array weights, where each element of the array, weights[i], represents the weight of the ith marble. Your goal is to distribute the marbles among the k bags based on specific rules to compute a score for each unique distribution. Then, you must determine the difference between the maximum and minimum scores possible among these distributions.

Rules for Distribution:

  • Each bag must contain at least one marble.
  • The marbles in a bag must be contiguous in the array. This means if marbles i and j are in a bag, all marbles between these indices must also be included in that same bag.
  • The cost of a bag containing marbles from index i to j (both inclusive) is calculated as the sum of the weights of the first and last marbles in that bag (weights[i] + weights[j]).

The overall score for a distribution of marbles is the sum of the costs of all bags. The result expected is the difference between the highest and lowest scores achievable under the given constraints.

Examples

Example 1

Input:

weights = [1,3,5,1], k = 2

Output:

4

Explanation:

The distribution [1],[3,5,1] results in the minimal score of (1+1) + (3+1) = 6.
The distribution [1,3],[5,1], results in the maximal score of (1+3) + (5+1) = 10.
Thus, we return their difference 10 - 6 = 4.

Example 2

Input:

weights = [1, 3], k = 2

Output:

0

Explanation:

The only distribution possible is [1],[3].
Since both the maximal and minimal score are the same, we return 0.

Constraints

  • 1 <= k <= weights.length <= 105
  • 1 <= weights[i] <= 109

Approach and Intuition

Given the problem's constraints and requirements, the approach involves exploring various ways to distribute the marbles into k bags such that each distribution yields a unique score. Here's how we can think about this:

  1. Understand the cost calculation for a single bag, which relies on the values of the first and last marbles in that bag.
  2. Every possible way to partition the weights into k contiguous subarrays represents a potential score.
  3. Using dynamic programming or a divide-and-conquer strategy may work effectively here since we need to compute the maximum and minimum possible scores and then their difference.
  4. Consider edge cases where the distribution results in the same score, yielding a difference of zero.
  5. The constraints suggest that a highly efficient algorithm might be necessary, especially for large arrays, as brute force could become computationally expensive.

By careful manipulation of the partitions and diligent calculation of costs for specific distributions, we can derive a range of scores and subsequently their maximum and minimum. This will allow us to determine the required difference.

Solutions

  • C++
cpp
class Solution {
public:
    long long distributeMarbles(vector<int>& marbleWeights, int count) {
        int totalMarbles = marbleWeights.size();
        vector<int> sumWeights(totalMarbles - 1, 0);
        for (int index = 0; index < totalMarbles - 1; ++index) {
            sumWeights[index] += marbleWeights[index] + marbleWeights[index + 1];
        }
    
        sort(sumWeights.begin(), sumWeights.end());
    
        long long result = 0;
        for (int j = 0; j < count - 1; ++j) {
            result += sumWeights[totalMarbles - 2 - j] - sumWeights[j];
        }
    
        return result;
    }
};

In the "Put Marbles in Bags" solution using C++, the given function distributeMarbles efficiently allocates marble weights into a certain number of bags, aiming for an optimal distribution based on the sum of weights. The solution accomplishes this through the following steps:

  1. Determine the total number of marbles using marbleWeights.size().
  2. Initialize a vector sumWeights to store cumulative weights of consecutive marbles.
  3. Loop through the marbleWeights array, summing weights of consecutive marbles and storing these in sumWeights.
  4. Sort the sumWeights vector in ascending order to facilitate optimal pair selection.
  5. Initialize a result accumulator (result) to store the final calculated difference between the selected weight sums.
  6. Iterate through the sorted sumWeights, from highest to lowest, accumulating differences between complementary sums in the array.
  7. Return the accumulated differences as the result of the function.

This method ensures a strategic distribution of weights, where differences in bag weights are minimized based on the sorting and selection approach. This approach manipulates the properties of sorted arrays to quickly achieve the desired result, applicable particularly in scenarios where bag count optimization based on weight is critical.

  • Java
java
class Solution {
    public long calculateDifference(int[] weights, int maxPairs) {
        int length = weights.length;
        int[] combinedWeights = new int[length - 1];
        for (int i = 0; i < length - 1; ++i) {
            combinedWeights[i] = weights[i] + weights[i + 1];
        }
            
        // Sorting the calculated sums for the defined range
        Arrays.sort(combinedWeights, 0, length - 1);
            
        long result = 0l;
        for (int i = 0; i < maxPairs - 1; ++i) {
            result += combinedWeights[length - 2 - i] - combinedWeights[i];
        }
    
        return result;
    }
}

The Java solution provided addresses the problem of finding the difference between the heaviest and lightest pairs from a list of weight sums. The process involves summing adjacent weights and then calculating the differences between the highest and lowest sums after sorting. Here's how the solution functions:

  • Start by calculating the sum of adjacent weights in the weights[] array and store these sums in a combinedWeights[] array.
  • Sort the combinedWeights[] array.
  • Calculate the total difference by considering sums from the higher-end and the lower-end of the combinedWeights[] array up to maxPairs - 1, ensuring the total pairs considered are one less than the provided maxPairs.
  • Return the resultant difference.

Apply these steps:

  1. Sum the weights of adjacent marbles.
  2. Store these sums in a new array.
  3. Sort this array of sums.
  4. Calculate the difference by subtracting the sorted array's smallest summed values from the largest, ensuring the count of the pairs considered aligns with the specified maximum pairs minus one.
  5. Return the total calculated difference as your result.

This approach guarantees that you efficiently find the desired difference based on the conditions specified, using simple array manipulations and sort operations.

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