881 Boats to Save People

Techniques Used

• Greedy
• Two Pointer
• Sorting

Approach

• Sort the people weights array.
• Now, use the two pointer technique to navigate the elements of people weights array.
• while the start is less than end index, do the following:
  • if adding the weight at the end index is under the prescribed limit AND number of people on the current boat is less than or equal to 1 ( people_on_curr_boat <= 1)
    • add the person at the end index and increment the number of people on the current boat ( people_on_curr_boat ) and increment the boat weight (curr_boat_size)
    • if boat_count was 0, then increment the boat_count. (Because we start with the boat_count of 0)
    • decrement the end Index.
  • else if (do the same above steps for the element at start index)
  • else
    • increment the boat_count. (as current boat is full and unable to accomodate further).
    • make the curr_boat_size as 0 because a new boat has been taken.
    • make the number of people on curr boat as 0. (people_on_curr_boat = 0)
• Once you are outside this while loop, just return the boat_count.

Complexity

• Time Complexity: O(nlogn + n)
  • logn for sorting.
  • n for visiting each element only once, and processing it.
• Space Complexity: O(1) - We are not using any space that grows with n.

Note: Please let me know if I am wrong anywhere in my analysis.

class Solution {
    public int numRescueBoats(int[] people, int limit) {


        Arrays.sort(people);

        int end = people.length - 1;
        int start = 0;

        int curr_boat_size = 0;
        int boat_count = 0;
        int people_on_curr_boat = 0;

        while(start <= end){


            if(curr_boat_size + people[end] <= limit && people_on_curr_boat <= 1) {
                people_on_curr_boat++;
                curr_boat_size += people[end];
                if(boat_count == 0) boat_count++;
                end--;
            }
            else if(curr_boat_size + people[start] <= limit && people_on_curr_boat <= 1){
                people_on_curr_boat++;
                curr_boat_size += people[start];
                if(boat_count == 0) boat_count++;
                start++;
            }
            else {
                boat_count++;
                curr_boat_size = 0;
                people_on_curr_boat = 0;

            }            

        }

        return boat_count;

    }
}