How To Put As Many Things As You Can On a Truck

C++ Solution to Leetcode 1710. Maximum Units on a Truck

Problem statement

You are assigned to put some amount of boxes onto one truck. You are given a 2D array boxTypes, where boxTypes[i] = [numberOfBoxes_i, numberOfUnitsPerBox_i]:

  • numberOfBoxes_i is the number of boxes of type i.
  • numberOfUnitsPerBox_i is the number of units in each box of the type i.

You are also given an integer truckSize, which is the maximum number of boxes that can be put on the truck. You can choose any boxes to put on the truck as long as the number of boxes does not exceed truckSize.

Return the maximum total number of units that can be put on the truck.

Example 1

Input: boxTypes = [[1,3],[2,2],[3,1]], truckSize = 4
Output: 8
Explanation: There are:
- 1 box of the first type that contains 3 units.
- 2 boxes of the second type that contain 2 units each.
- 3 boxes of the third type that contain 1 unit each.
You can take all the boxes of the first and second types, and one box of the third type.
The total number of units will be = (1 * 3) + (2 * 2) + (1 * 1) = 8.

Example 2

Input: boxTypes = [[5,10],[2,5],[4,7],[3,9]], truckSize = 10
Output: 91
Explanation: (5 * 10) + (3 * 9) + (2 * 7) = 91.


  • 1 <= boxTypes.length <= 1000.
  • 1 <= numberOfBoxes_i, numberOfUnitsPerBox_i <= 1000.
  • 1 <= truckSize <= 10^6.

Solution: Greedy algorithm

Put the boxes having more units first.


#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int maximumUnits(vector<vector<int>>& boxTypes, int truckSize) 
    // sort for the boxes based on their number of units
    sort(boxTypes.begin(), boxTypes.end(), 
        [](const vector<int>& a, const vector<int>& b) 
            return a[1] > b[1];
    int maxUnits = 0;
    int i = 0;
    while (truckSize > 0 && i < boxTypes.size()) 
        if (boxTypes[i][0] <= truckSize) 
            maxUnits += boxTypes[i][0] * boxTypes[i][1];
            truckSize -= boxTypes[i][0];
            maxUnits += truckSize * boxTypes[i][1];
    return maxUnits;
int main() 
    vector<vector<int>> boxTypes{{1,3},{2,2},{3,1}};
    cout << maximumUnits(boxTypes, 4) << endl;
    boxTypes = {{5,10},{2,5},{4,7},{3,9}};
    cout << maximumUnits(boxTypes, 10) << endl;

Code explanation

  1. The boxTypes vector is sorted in descending order based on the number of units per box. This is done using a lambda function as the sorting criterion. Sorting in this way ensures that the boxes with the most units per box come first in the sorted list.

  2. The code initializes maxUnits to 0. This variable will keep track of the maximum units that can be loaded onto the truck.

  3. It initializes an index variable i to 0, which will be used to iterate through the sorted boxTypes list.

  4. The code enters a loop that continues as long as there is available space on the truck (truckSize > 0) and there are more box types to consider (i < boxTypes.size()).

  5. It checks if the current box type (specified by boxTypes[i]) can be loaded entirely onto the truck without exceeding its capacity. If boxTypes[i][0] (the number of boxes available for this type) is less than or equal to truckSize, it loads all available boxes of this type onto the truck:

    • It adds boxTypes[i][0] * boxTypes[i][1] to maxUnits (the total units loaded).
    • It subtracts boxTypes[i][0] from truckSize (reduce the remaining truck capacity).
  6. If the current box type cannot be loaded entirely onto the truck without exceeding its capacity, it loads as many boxes as possible to maximize the units:

    • It adds truckSize * boxTypes[i][1] to maxUnits (the total units loaded).
    • It breaks out of the loop since the truck is now at full capacity.
  7. The code increments i to move to the next box type.

  8. After the loop finishes, maxUnits will contain the maximum number of units that can be loaded onto the truck without exceeding its capacity.

  9. The code returns the value of maxUnits as the result.

In summary, the code optimally loads boxes onto a truck to maximize the total number of units that can be transported, considering both the number of boxes available and their units per box.


  • Runtime: O(N*logN), where N = boxTypes.length.
  • Extra space: O(1).

Modern C++ STL notes

Note that two vectors can be compared. That is why you can sort them.

But in this case you want to sort them based on the number of units. That is why you need to define the comparison function like the code above. Otherwise, the sort algorithm will use the dictionary order to sort them by default.

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