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description:

A conveyor belt has packages that must be shipped from one port to another within days days.

The ith package on the conveyor belt has a weight of weights[i]. Each day, we load the ship with packages on the conveyor belt (in the order given by weights). We may not load more weight than the maximum weight capacity of the ship.

Return the least weight capacity of the ship that will result in all the packages on the conveyor belt being shipped within days days.

Example 1:

Input: weights = [1,2,3,4,5,6,7,8,9,10], days = 5
Output: 15
Explanation: A ship capacity of 15 is the minimum to ship all the packages in 5 days like this:
1st day: 1, 2, 3, 4, 5
2nd day: 6, 7
3rd day: 8
4th day: 9
5th day: 10

Note that the cargo must be shipped in the order given, so using a ship of capacity 14 and splitting the packages into parts like (2, 3, 4, 5), (1, 6, 7), (8), (9), (10) is not allowed.
Example 2:

Input: weights = [3,2,2,4,1,4], days = 3
Output: 6
Explanation: A ship capacity of 6 is the minimum to ship all the packages in 3 days like this:
1st day: 3, 2
2nd day: 2, 4
3rd day: 1, 4
Example 3:

Input: weights = [1,2,3,1,1], days = 4
Output: 3
Explanation:
1st day: 1
2nd day: 2
3rd day: 3
4th day: 1, 1

Constraints:

1 <= days <= weights.length <= 5 * 104
1 <= weights[i] <= 500

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solution

/**
 * @param {number[]} weights
 * @param {number} days
 * @return {number}
 */
var shipWithinDays = function(weights, D) {
    function getDays(capacity) {
        let days = 1, total = 0;

        // See how many days it will take to unload all the weights given the current capacity
        for(let n of weights) {
            total += n;
            // Increase days only when we will exceed the capacity
            if(total > capacity) {
                total = n;
                days++;
            }
        }
        return days;
    }

    // Start is the minimum capacity of one day which is the max of all the weights
    // End is the max capacity we could load on one day which is the sum of all the weights
    let start = Math.max(...weights), 
        end = weights.reduce((acc, cur) => acc + cur, 0);

    // The min cacpaity needed is between start and finish
    while(start < end) {
        const mid = Math.floor((end+start)/2);
        // See how many days it would take to unload all the weigths given mid as the current capacity
        const days = getDays(mid);
        // If capacity at mid took more than D days to load, then we can add more weigth on the ships each day which means we need to increase the daily capacity
        if(days > D) start = mid+1;
        // mid might be our answer so we cannot set end to mid-1
        else end = mid;
    }
    return end;
};

leetcode

challenge

Here is the link for the problem:
https://leetcode.com/problems/capacity-to-ship-packages-within-d-days/