leetcode-1021-easy

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 = P1 + P2 + ... + Pk, where Pi 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: s = "(()())(())"
Output: "()()()"
Explanation: 
The input string is "(()())(())", with primitive decomposition "(()())" + "(())".
After removing outer parentheses of each part, this is "()()" + "()" = "()()()".
Example 2:

Input: s = "(()())(())(()(()))"
Output: "()()()()(())"
Explanation: 
The input string is "(()())(())(()(()))", with primitive decomposition "(()())" + "(())" + "(()(()))".
After removing outer parentheses of each part, this is "()()" + "()" + "()(())" = "()()()()(())".
Example 3:

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

1 <= s.length <= 105
s[i] is either '(' or ')'.
s is a valid parentheses string.

思路一：字符自会出现两种情况，当出现 ( 字符时，只要检查一下前面出现的左右字符数目是否相等，就可以把前面的字符去除最外层的括号

    public String removeOuterParentheses(String s) {
        Deque<Character> stack = new ArrayDeque<>();

        char[] chars = s.toCharArray();
        StringBuilder sb = new StringBuilder();
        int left = 0;
        int right = 0;
        for (char c : chars) {
            if (c == '(' && left == right && left > 0) {
                sb.append(getInnerString(stack));
            } else {
                stack.push(c);
            }

            if (c == '(') {
                left++;
            } else {
                right++;
            }
        }

        sb.append(getInnerString(stack));

        return sb.toString();
    }

    private String getInnerString(Deque<Character> stack) {
        StringBuilder sb = new StringBuilder();

        stack.pop();
        while (stack.size() > 1) {
            sb.insert(0, stack.pop());
        }

        return sb.toString();
    }