Encode and Decode Strings

https://www.lintcode.com/problem/659/

class Solution:
    """
    @param: strs: a list of strings
    @return: encodes a list of strings to a single string.
    """
    def encode(self, strs):
        # write your code here
        res = ""
        n = len(strs)
        for i in range(n-1):
            res += strs[i] + ":;"
        res += strs[n-1]
        return res
    """
    @param: str: A string
    @return: dcodes a single string to a list of strings
    """
    def decode(self, str):
        # write your code here
        print(str)
        res = []
        slow, fast = 0, 0
        while fast < len(str):
            if str[fast-1] == ":" and str[fast] == ";":
                res.append(str[slow:fast-1])
                slow = fast + 1
            fast += 1
        res.append(str[slow:])
        return res