The Problem

You are given a dictionary of words, followed by a list of sentences which have been stripped of all punctuation. You are to determine if spaces can be re-inserted into the sentences such that each word created appears in the given dictionary.

Things to consider

  • Words may overlap
    • i.e. “cat” and “catch”
    • Where you choose to insert the space is important in this case
    • This prevents heuristic greedy algorithms
  • A word may appear more than once in any sentence
  • Test cases are large enough that computing all possible dictionary combinations will eat my CPU

Things to consider

  • The key to this problem is finding all valid word prefixes for a given sentence
  • Since words may overlap, we must explore each one


  • Dictionary:
    • cat
    • catch
    • cha
  • Sentence:
    • catcha
  • You must consider the case of [catch][a] and [cat][cha]

Data Representation

  • Since we will be looking for valid word prefixes, this problem lends itself well to the Trie data structure (also known as a prefix tree)
  • Trie’s are ordered tree data structures which have the property that all descendants of a given node share a common prefix
  • Used commonly for auto-complete and text prediction


  • Like graphs, most languages do not include a trie structure in the standard library
  • Again though, they are easily created using existing data structures
  • Each node in the trie contains a map of a Character => TrieNode denoting an out going edge labeled with the character incident on the given TrieNode
  • For the purpose of this problem we only need a minimal trie implementation
  • insert, and prefixMatches are all that are necessary
  • Note that tries can also be used to implement pattern matching i.e.
    • find all strings that start with a, followed by any 3 characters, and end with b
    • These features are used for things like text prediction and auto complete

Trie Node Implementation

Insert Function

Finding matching prefixes

Putting it all together

  • Now that we have a way of finding all valid prefixes, we just have to design an algorithm to solve the problem
  • Basically, for each valid prefix we find, we want to remove it from the sentence and then repeat the process with the remaining string
  • Once we have exhausted the entire input string, we know that it was a valid sentence
  • Seems like a recursive problem

The algorithm

Full solution