LeetCode 1087. Brace Expansion



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.

Example 1:

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

Example 2:

Input: s = "abcd"
Output: ["abcd"]


  • 1 <= s.length <= 50
  • s consists of curly brackets '{}', commas ',', and lowercase English letters.
  • s is guaranteed to be a valid input.
  • There are no nested curly brackets.
  • All characters inside a pair of consecutive opening and ending curly brackets are different.


The goal of the problem is to find all combination strings. Whenever we encounter a ‘{‘, we can build combinations from different options, after that, we go to the character after ‘}’ to continue building combinations. When we encounter a letter, we just go to the next character in the string. In the end, sort the results in alphabetically ascending order.

Python Solution

class Solution:
    def expand(self, s: str) -> List[str]:
        results = []
        end = len(s)
        self.helper(results, "", 0, s)
        results = sorted(results)
        return results
    def helper(self, results, combination, start, s):        
        if start == len(s):
        i = start
        if s[i] == '{':
            j = i + 1            
            options = []

            while s[j] != '}':
                if s[j] != ',':
                j += 1
            for option in options:                
                self.helper(results, combination + option, j + 1, s)

        elif s[i] == '}':
            self.helper(results, combination + s[i], i + 1, s)
  • Time Complexity: O(N).
  • Space Complexity: O(N).

where Q is the length of queries and N is the length of colors.

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