LeetCode394：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".

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LeetCode：链接

看到括号匹配的题肯定想到用栈去做。

这个题，遇到’[‘就把之前的字符串进行进栈操作。遇到’]'进行出栈操作。curstring保存的是出栈操作完成后的字符串。注意这一步：curstring = prestring + prenum * curstring，prestring是前面的字符串，prenum * curstring是这一步骤结束之后的字符串，所以是前面的字符串+现在的字符串得到目前已有的字符串。

class Solution(object):
    def decodeString(self, s):
        """
        :type s: str
        :rtype: str
        """
        stack = []
        curstring = ''
        curnum = 0
        for char in s:
            if char == '[':
                stack.append(curstring)
                stack.append(curnum)
                # 之前的进栈之后就重头开始
                curstring = ''
                curnum = 0
            elif char == ']':
                prenum = stack.pop()
                prestring = stack.pop()
                curstring = prestring + curstring * prenum
            elif char.isdigit():
                # 如果是"100[leetcode]" 就必须用10 * curnum + int(char)
                curnum = 10 * curnum + int(char)
            else:
                curstring += char
        return curstring