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.

Examples:

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


## Solutions

### 1. 递归

采用递归的方法，主要得分析清楚怎么去找 start 和 end。

# Time Complexity: O(n^2)
# Space Complexity: O(n)
class Solution:
def decodeString(self, s: str) -> str:
if not s or len(s) == 0:
return ''

if not s.isdigit():
return s + self.decodeString(s[1:])

loop = 0
start, end = 0, 0
n = len(s)
#  3   [   a   ]   2   [   b   c   ]
#    start    end
for i in range(n):
if s[i] == '[':
if loop == 0:
start = i
loop += 1
if s[i] == ']':
loop -= 1
if loop == 0:
end = i
break

res = ''
# repeat substring
times = int(s[:start])
for i in range(times):
res += self.decodeString(s[start+1:end])
return res + self.decodeString(s[end+1:])
# Runtime: 28 ms, faster than 97.31% of Python3 online submissions for Decode String.
# Memory Usage: 12.7 MB, less than 100.00% of Python3 online submissions for Decode String.


### 2. Stack

当然也可以用两个 stack 来实现，分别存数字和字串，这里用一个 stack 也能实现同样的功能。

# Time Complexity: O(n)
# Space Complexity: O(n)
class Solution:
def decodeString(self, s: str) -> str:
stack = []
curnum, curstr = 0, ''
for c in s:
if c.isdigit():
curnum = curnum * 10 + int(c)
elif c == '[':
stack.append(curnum)
stack.append(curstr)
curstr = ''
curnum = 0
elif c == ']':
prestr = stack.pop()
prenum = stack.pop()
curstr = prestr + curstr * prenum
else:
curstr += c
return curstr

# Runtime: 28 ms, faster than 97.31% of Python3 online submissions for Decode String.
# Memory Usage: 12.8 MB, less than 100.00% of Python3 online submissions for Decode String.