#1087 - Brace Expansion

Problem Description

You are given a string s representing a list of words. Each letter in the word has one or more options.

If there is one option, the letter is represented as is.
If there is more than one option, then curly braces delimit the options. For example, "{a,b,c}" represents options ["a", "b", "c"].

For example, if s = "a{b,c}", the first character is always 'a', but the second character can be 'b' or 'c'. The original list is ["ab", "ac"].

Return all words that can be formed in this manner, sorted in lexicographical order.

Solution

/**
 * @param {string} s
 * @return {string[]}
 */
var expand = function(s) {
  const groups = [];
  let i = 0;

  while (i < s.length) {
    if (s[i] === '{') {
      const start = i + 1;
      while (s[i] !== '}') i++;
      const options = s.substring(start, i).split(',');
      groups.push(options);
      i++;
    } else {
      groups.push([s[i]]);
      i++;
    }
  }

  const result = [];

  backtrack(0, '');

  return result.sort();

  function backtrack(index, current) {
    if (index === groups.length) {
      result.push(current);
      return;
    }

    for (const option of groups[index]) {
      backtrack(index + 1, current + option);
    }
  }
};