1021. Remove Outermost Parentheses

Tags: ‘Stack’

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:

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

Solution

Refer to 20.Valid Parenthesis

public String removeOuterParentheses(String S) {
    Deque<Character> stack = new LinkedList<>();
    List<String> list = new ArrayList<>();
    int currentIndex = 0;
    for (int i = 0; i < S.length(); i++) {
        char c = S.charAt(i);
        if (c == '(') {
            stack.push(c);
        } else if (c == ')' && !stack.isEmpty()) {
            stack.pop();
        }
        
        if (stack.isEmpty()) {
            // found a primitive
            list.add(S.substring(currentIndex, i + 1));
            currentIndex = i + 1;
        }
    }
    
    String result = "";
    for (String s: list) {
        result += s.substring(1, s.length() - 1);
    }
    return result;
}

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1021. Remove Outermost Parentheses