LeetCode-in-All
394. Decode String
Medium
Given an encoded string, return its decoded string.
The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.
You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.
Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there won’t be input like 3a or 2[4].
Example 1:
Input: s = “3[a]2[bc]”
Output: “aaabcbc”
Example 2:
Input: s = “3[a2[c]]”
Output: “accaccacc”
Example 3:
Input: s = “2[abc]3[cd]ef”
Output: “abcabccdcdcdef”
Example 4:
Input: s = “abc3[cd]xyz”
Output: “abccdcdcdxyz”
Constraints:
1 <= s.length <= 30sconsists of lowercase English letters, digits, and square brackets'[]'.sis guaranteed to be a valid input.- All the integers in
sare in the range[1, 300].
Solution
# @param {String} s
# @return {String}
def decode_string(s)
@i = 0
decode_helper(s)
end
private
def decode_helper(s)
count = 0
result = ""
while @i < s.length
c = s[@i]
@i += 1
if letter?(c)
result += c
elsif digit?(c)
count = count * 10 + c.to_i
elsif c == ']'
break
elsif c == '['
# sub problem
repeat = decode_helper(s)
result += repeat * count
count = 0
end
end
result
end
def letter?(c)
c =~ /[a-zA-Z]/
end
def digit?(c)
c =~ /\d/
end