19. Minimum Remove to Make Valid Parentheses.cpp

/*
Minimum Remove to Make Valid Parentheses
==========================================

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)"

Constraints:
1 <= s.length <= 10^5
s[i] is one of  '(' , ')' and lowercase English letters.

Hint #1  
Each prefix of a balanced parentheses has a number of open parentheses greater or equal than closed parentheses, similar idea with each suffix.

Hint #2  
Check the array from left to right, remove characters that do not meet the property mentioned above, same idea in backward way.
*/

class Solution
{
public:
  string minRemoveToMakeValid(string s)
  {
    stack<int> pos;
    for (int i = 0; i < s.size(); ++i)
    {
      if (s[i] == '(')
        pos.push(i);
      else if (s[i] == ')')
      {
        if (pos.size())
          pos.pop();
        else
          s[i] = '*';
      }
    }

    while (pos.size())
    {
      s[pos.top()] = '*';
      pos.pop();
    }

    string ans = "";
    for (auto &i : s)
    {
      if (i != '*')
        ans += i;
    }
    return ans;
  }
};