Formatted question description: https://leetcode.ca/all/1021.html

1021. Remove Outermost Parentheses (Easy)

A valid parentheses string is either empty (""), "(" + A + ")", or A + B, where A and B are valid parentheses strings, and + represents string concatenation.  For example, "", "()", "(())()", and "(()(()))" are all valid parentheses strings.

A valid parentheses string S is primitive if it is nonempty, and there does not exist a way to split it into S = A+B, with A and B nonempty valid parentheses strings.

Given a valid parentheses string S, consider its primitive decomposition: S = P_1 + P_2 + ... + P_k, where P_i are primitive valid parentheses strings.

Return S after removing the outermost parentheses of every primitive string in the primitive decomposition of S.

 

Example 1:

Input: "(()())(())"
Output: "()()()"
Explanation: 
The input string is "(()())(())", with primitive decomposition "(()())" + "(())".
After removing outer parentheses of each part, this is "()()" + "()" = "()()()".

Example 2:

Input: "(()())(())(()(()))"
Output: "()()()()(())"
Explanation: 
The input string is "(()())(())(()(()))", with primitive decomposition "(()())" + "(())" + "(()(()))".
After removing outer parentheses of each part, this is "()()" + "()" + "()(())" = "()()()()(())".

Example 3:

Input: "()()"
Output: ""
Explanation: 
The input string is "()()", with primitive decomposition "()" + "()".
After removing outer parentheses of each part, this is "" + "" = "".

 

Note:

  1. S.length <= 10000
  2. S[i] is "(" or ")"
  3. S is a valid parentheses string
 

Companies:
Google

Related Topics:
Stack

Solution 1.

// OJ: https://leetcode.com/problems/remove-outermost-parentheses/

// Time: O(N)
// Space: O(1)
class Solution {
public:
    string removeOuterParentheses(string S) {
        string ans;
        int cnt = 0, start = 0;
        for (int i = 0; i < S.size(); ++i) {
            if (S[i] == '(') ++cnt;
            else if (--cnt == 0) {
                ans += S.substr(start + 1, i - start - 1);
                start = i + 1;
            }
        }
        return ans;
    }
};

Java

class Solution {
    public String removeOuterParentheses(String S) {
        List<Integer> indices = new ArrayList<Integer>();
        Stack<Character> stack = new Stack<Character>();
        int length = S.length();
        int begin = 0;
        for (int i = 0; i < length; i++) {
            char c = S.charAt(i);
            if (c == '(')
                stack.push(c);
            else {
                stack.pop();
                if (stack.isEmpty()) {
                    indices.add(begin);
                    indices.add(i);
                    begin = i + 1;
                }
            }
        }
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < length; i++) {
            if (!indices.contains(i))
                sb.append(S.charAt(i));
        }
        return sb.toString();
    }
}

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