Maximum Units on a Truck

"""
Approach (Greedy / Fractional Knapsack):
To maximize total units/value with a limited capacity `n`, always pick from the item
that gives the highest units per box (value density) first. So we sort `arr` in
descending order of `unitsPerBox`, then take as many boxes as possible from each item.
If the remaining capacity is smaller than available boxes, we take only that fraction
and stop. This guarantees the maximum possible total units.
"""


class Solution:
    def maximumUnits(self, arr: List[List[int]], n: int) -> int:
        arr.sort(key=lambda x: x[1],reverse=True)
        ans=0
        i=0
        while i<len(arr) and n>0:
            if n>arr[i][0]:
                ans+=(arr[i][0]*arr[i][1])
                n-= arr[i][0]
                i+=1
            else:
                ans+=(n*arr[i][1])
                n=0
                i+=1
        return ans