Formatted question description: https://leetcode.ca/all/1249.html

1249. Minimum Remove to Make Valid Parentheses (Medium)

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.

Related Topics:
String, Stack

Solution 1. Scan Left to Right then Right to Left

// OJ: https://leetcode.com/problems/minimum-remove-to-make-valid-parentheses/

// Time: O(N)
// Space: O(1)
class Solution {
public:
    string minRemoveToMakeValid(string s) {
        int left = 0, i, j;
        for (i = 0, j = 0; i < s.size(); ++i) {
            if (isalpha(s[i]) || (s[i] == '(' && ++left) || (s[i] == ')' && --left >= 0)) s[j++] = s[i];
            left = max(left, 0);
        }
        s.resize(j);
        reverse(begin(s), end(s));
        left = 0;
        for (i = 0, j = 0; i < s.size(); ++i) {
            if (isalpha(s[i]) || (s[i] == ')' && ++left) || (s[i] == '(' && --left >= 0)) s[j++] = s[i];
            left = max(left, 0);
        }
        s.resize(j);
        reverse(begin(s), end(s));
        return s;
    }
};

Solution 2. Stack

// OJ: https://leetcode.com/problems/minimum-remove-to-make-valid-parentheses/

// Time: O(N)
// Space: O(N)
class Solution {
public:
    string minRemoveToMakeValid(string s) {
        stack<int> st;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] == '(') st.push(i);
            else if (s[i] == ')') {
                if (st.empty()) s[i] = '*';
                else st.pop();
            }
        }
        while (st.size()) {
            s[st.top()] = '*';
            st.pop();
        }
        s.erase(remove(begin(s), end(s), '*'), end(s));
        return s;
    }
};

Java

class Solution {
    public String minRemoveToMakeValid(String s) {
        Stack<Integer> stack = new Stack<Integer>();
        StringBuffer sb = new StringBuffer();
        int length = s.length();
        int deleteCount = 0;
        for (int i = 0; i < length; i++) {
            char c = s.charAt(i);
            if (Character.isLetter(c))
                sb.append(c);
            else if (c == '(') {
                sb.append(c);
                stack.push(i - deleteCount);
            } else if (c == ')') {
                if (!stack.isEmpty()) {
                    sb.append(c);
                    stack.pop();
                } else
                    deleteCount++;
            }
        }
        while (!stack.isEmpty()) {
            int index = stack.pop();
            sb.deleteCharAt(index);
        }
        return sb.toString();
    }
}

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