Home > Applied Algorithms, Dynamic Programming > Santa Claus is coming to town. (Integer Knapsack 1/0)

Santa Claus is coming to town. (Integer Knapsack 1/0)

I hope you remember the US Pizza problem we talked about a while back. That wasn’t a 1/0 problem but the one we will discuss now certainly is.

Problem Statement: Santa Claus is coming to your city and needs to figure out the gifts he has to take. Since North Pole is a long way off, he can only carry gifts weighing a total of ‘C’ in his bag. He has a total of ‘N’ gifts to select from. You are supposed to help him select the gifts in a way that would use the capacity to the greatest use, i.e., the total weight of the gifts selected should be closest to C and the most number of gifts should be selected.

This is a dynamic programming problem and you should recognize it as soon as you see that the problem has optimal substructures in the fact that its solution can be built starting from 1 to i gifts. Also, this is a 1/0 knapsack problem since you can either select a gift (1) or leave it behind (0).

Let us define a term M(i,j) as the optimal way of selecting gifts in a way such that selecting 1..i gifts brings the total weight up to exactly j. So your value of i varies from 1 to N and the value of j varies from 0 to C. Hence, the time complexity of the problem is equal to O(NC). (Basically, M(i,j) in a 2D array stores the number of gifts)

At every point, there are two ways in which the value of M(i,j) can be determined.

1. Check if there was a subset of gifts from number 1 to i-1 which formed a subset with total weight equal to j. You have the value M(i-1,j) as one of your candidate values.

2. Check the M(i-1, j-Wi) value such that adding the ith gifts weight gives us j. (i.e, weight of ith gift is Wi). This value plus 1 (because we pick up this ith gift as well) is another candidate value.

Now you simply have to take the maximum of these two values and place the value at M(i,j). In this way, you end up finding M(n,C) in the generated 2D array which is the required answer that our Santa Claus requires.

Here, you should notice how we built the solution for the problem in a manner similar to what we might do for a balanced partition problem. I will discuss this problem in the next post.

Stay tuned for more Dynamic Programming solutions. 🙂

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