Remove Outermost Parentheses LeetCode Solution - The Coding Shala

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 In this post, we will learn how to solve LeetCode's Remove Outermost Parentheses problem and will implement its solution in Java.

Remove Outermost Parentheses Problem

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.

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 "()()" + "()" + "()(())" = "()()()()(())".

Practice this problem on LeetCode.

LeetCode - Remove Outermost Parentheses Java Solution

Approach 1

Iterative Solution.

Java Program: 

class Solution {
    public String removeOuterParentheses(String S) {
        int cnt = 0;
        StringBuilder sb = new StringBuilder();
        for(int i=0; i<S.length(); i++) {
            if(cnt == 0) {
                cnt++;
            } else if(cnt == 1 && S.charAt(i) == ')') {
                cnt--;
            } else {
                sb.append(S.charAt(i));
                if(S.charAt(i) == '(') cnt++;
                else cnt--;
            }
        }
        return sb.toString();
    }
}

Approach 2

Using Stack.

Java Program: 

class Solution {
    public String removeOuterParentheses(String S) {
        Stack<Character> stack = new Stack<>();
        StringBuilder ans = new StringBuilder();
        for(int i=0; i<S.length(); i++) {
            char ch = S.charAt(i);
            if(ch == '(') {
                if(stack.size() > 0) {
                    ans.append(ch);
                }
                stack.push(ch);
            } else {
                if(stack.size() > 1) {
                    ans.append(ch);
                }
                stack.pop();
            }
        }
        return ans.toString();
    }
}

Here we don't need a stack, by using count we can easily implement this solution.

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