1249. Minimum Remove to Make Valid Parentheses.py

1249. Minimum Remove to Make Valid Parentheses
Medium/1286/32

Given a string s of '(' , ')' and lowercase English characters. 

Your task is to remove the minimum number of parentheses ( '(' or ')', in any positions ) 
so that the resulting parentheses string is valid and return any valid string.

Formally, a parentheses string is valid if and only if:

It is the empty string, contains only lowercase characters, or
It can be written as AB (A concatenated with B), where A and B are valid strings, or
It can be written as (A), where A is a valid string.
 

Example 1:

Input: s = "lee(t(c)o)de)"
Output: "lee(t(c)o)de"
Explanation: "lee(t(co)de)" , "lee(t(c)ode)" would also be accepted.
Example 2:

Input: s = "a)b(c)d"
Output: "ab(c)d"

Example 3:

Input: s = "))(("
Output: ""
Explanation: An empty string is also valid.

Example 4:

Input: s = "(a(b(c)d)"
Output: "a(b(c)d)"

class Solution(object):
	def minRemoveToMakeValid(self, s):
		"""
		:type s: str
		:rtype: str
		"""
		sb = []
		for char in s:
			sb.append(char)
		st = []
		for i in range(len(s)):
			if s[i] == '(':
				st.append(i)
			if s[i] == ')':
				if st:
					st.pop()
				else:
					sb[i] = '*'
		while st:
			sb[st.pop()] = '*'
		for i in range(len(sb)):
			if sb[i] == '*': 
				sb[i] = ''
		return ''.join(sb)

time: o(n)
space: o(n)