LeetCode #394 Decode String

Medium

Problem

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

Use two stacks to save currently tracing snippet and how many times this snippet should be repeated. Once we met a [, push current snippet and count into stacks. Or if a ] is encountered, pop the count out and evaluate decoded string of current snippet.

Complexity

It’s trivial that it takes O(n) time to iterate entire string, where n denotes to length of the given string.

For space complexity, it will use O(m) extra space for saving entire decoded string, where m denotes to the length of the result string.