Brace Expansion

A string S represents a list of words.

Each letter in the word has 1 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, "{a,b,c}d{e,f}" represents the list ["ade", "adf", "bde", "bdf", "cde", "cdf"].

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

Example 1:

Input: "{a,b}c{d,e}f"
Output: ["acdf","acef","bcdf","bcef"]

Example 2:

Input: "abcd"
Output: ["abcd"]


  1. 1 <= S.length <= 50
  2. There are no nested curly brackets.
  3. All characters inside a pair of consecutive opening and ending curly brackets are different.
class Solution:
    def expand(self, S):

        def func(remain, result, results):
            if not remain:

                # if a single character appear at the first of the remain string
                if remain[0] != "{":
                    func(remain[1:], result + remain[0], results)
                    temp = list()
                    i = 0

                    # find elements in the first brace of the remain string
                    while remain[i] != "}":
                        if remain[i].isalpha():
                        i += 1

                    for t in temp:
                        func(remain[i + 1:], result + t, results)

        results = []

        func(S, "", results)

        return results

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