1021. Remove Outermost Parentheses

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 "" + "" = "".

解题要点：

count左右括号出现次数，如左，count加一，如右，count减少一。如count等于0时，先取出这段有效括号，再处理去掉外层括号，加进StringBuffer，最后更新下一次的起点。返回sb.toString()。

class Solution {
    public String removeOuterParentheses(String S) {
        int count = 0, start = 0;
        StringBuffer sb = new StringBuffer();
        for(int i = 0; i < S.length(); i++){
            if(S.charAt(i)== '(') count++;
            else if(S.charAt(i) == ')') count--;
            
            if(count == 0){
                String temp = S.substring(start, i + 1);
                temp = temp.substring(1, temp.length() - 1);
                sb.append(temp);
                start = i + 1;
            }
        }
        return sb.toString();
    }
}