problem_259.js

// This problem was asked by Two Sigma.

// Ghost is a two-person word game where players alternate appending letters to a word. The first person who spells out a word, or creates a prefix for which there is no possible continuation, loses. Here is a sample game:

// Player 1: `g`
// Player 2: `h`
// Player 1: `o`
// Player 2: `s`
// Player 1: `t` [loses]

// Given a dictionary of words, determine the letters the first player should start with, such that with optimal play they cannot lose.

// For example, if the dictionary is `["cat", "calf", "dog", "bear"]`, the only winning start letter would be `b`.