Add knapsack problem.

Author: trekhleb

README.md

@@ -40,6 +40,7 @@
   * [Longest Common Subsequence](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/longest-common-subsequnce) (LCS) 
   * [Longest Increasing subsequence](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/longest-increasing-subsequence)
   * [Shortest Common Supersequence](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/shortest-common-supersequence) (SCS)
+  * [Knapsack Problem](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/knapsack-problem)
 * **String**
   * [Levenshtein Distance](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/levenshtein-distance) - minimum edit distance between two sequences
   * [Hamming Distance](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/hamming-distance) - number of positions at which the symbols are different
@@ -94,7 +95,7 @@
   * [Longest Common Substring](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/longest-common-substring)
   * [Longest Increasing subsequence](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/longest-increasing-subsequence)
   * [Shortest Common Supersequence](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/shortest-common-supersequence)
-  * Knapsack problem
+  * [Knapsack Problem](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/knapsack-problem)
   * Maximum subarray
   * Maximum sum path
   * Integer Partition

src/algorithms/sets/knapsack-problem/README.md

@@ -0,0 +1,23 @@
+# Knapsack Problem
+
+The knapsack problem or rucksack problem is a problem in 
+combinatorial optimization: Given a set of items, each with 
+a weight and a value, determine the number of each item to 
+include in a collection so that the total weight is less 
+than or equal to a given limit and the total value is as 
+large as possible.
+
+It derives its name from the problem faced by someone who is 
+constrained by a fixed-size knapsack and must fill it with the 
+most valuable items.
+
+Example of a one-dimensional (constraint) knapsack problem: 
+which boxes should be chosen to maximize the amount of money 
+while still keeping the overall weight under or equal to 15 kg?
+
+![knapsack problem](https://upload.wikimedia.org/wikipedia/commons/f/fd/Knapsack.svg)
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Knapsack_problem)
+- [YouTube](https://www.youtube.com/watch?v=8LusJS5-AGo)