394. Decode String

Given an encoded string, return it's 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].

Examples:

s = "3[a]2[bc]", return "aaabcbc".
s = "3[a2[c]]", return "accaccacc".
s = "2[abc]3[cd]ef", return "abcabccdcdcdef".

Solution: Two Stacks

class Solution(object):
    def decodeString(self, s):
        """
        :type s: str
        :rtype: str
        """
        nums = []
        subs = [[]]
        idx = 0
        while idx < len(s):
            char = s[idx]
            if char.isdigit():
                bracket = s.find('[', idx)
                nums.append(int(s[idx: bracket]))
                subs.append([])
                idx = bracket
            elif char == ']':
                num = nums.pop()
                sub = subs.pop()
                subs[-1].extend(sub * num)
            else:
                subs[-1].append(char)
            idx += 1
        return ''.join(subs.pop())

Lessons:

• Use two stacks, one for numbers, and one for sub-strings.
• Use nested stack for sub-strings.
• Every time see a [, push a new empty stack to sub-string stack.
• Every time see a ], pop the last stack in sub-string stack, repeat as needed, then merge into its previous neighbor.