LeetCode解题分享：394. Decode String

Problem

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].

Examples:

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

解题思路

   这一题的基本思路是使用递归（或者栈）来模拟解码的过程。在这里，我们采用递归下降分析的方法来进行求解。递归下降分析是编译器设计中使用的一种技术，本质上是使用递归的方法来进行编程语言的语法分析。只要事先设计好合适的状态转移，就可以很好地进行模拟。

   代码如下：

class Solution:
    def decodeString(self, s: str) -> str:
        pos = 0

        def func():
            nonlocal pos
            result = ""
            while 1:
                while pos < len(s) and ('a' <= s[pos] <= 'z' or 'A' <= s[pos] <= 'Z'):
                    result += s[pos]
                    pos += 1

                num = 0

                r = ""
                while pos < len(s) and '0' <= s[pos] <= '9':
                    num = num * 10 + int(s[pos])
                    pos += 1

                if pos < len(s) and s[pos] == '[':
                    pos += 1
                    r = func()

                    if pos < len(s) and s[pos] == ']':
                        pos += 1
                result += r * num

                if pos == len(s) or s[pos] == ']':
                    return result

        return func()