How to remove the outermost parentheses in php

php method to remove the outermost parentheses: first perform primitive decomposition on the non-empty valid string S; then obtain the valid bracket string primitive; then delete and decompose the most primitive string of each primitive string Outer brackets; finally return the string S.

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php method to remove the outermost parentheses:

The valid bracket string is empty (""), "(" A ")" or A B, where A and B are both is a valid bracketed string, representing the concatenation of strings. For example, "", "()", "(())()" and "(()(()))" are all valid bracket strings.

If the valid string S is non-empty and there is no way to split it into S = A B, we call it a primitive, where A and B are both non-empty valid bracket strings.

Given a non-empty valid string S, consider decomposing it into primitives such that: S = P_1 P_2 ... P_k, where P_i is a valid bracket string primitive.

Perform primitive decomposition on S, delete the outermost brackets of each primitive string in the decomposition, and return S.

Example 1:

Input: "(()())(())"

Output: "()()()"

Explanation:

The input string is "(()())(())", and the primitive decomposition is "(() ())" "(())",

After deleting the outermost bracket in each part, we get "()()" "()" = "()()()".

Example 2:

Input: "(()())(())(()(()))"

Output: "()()()()(())"

Explanation:

The input string is "(()())(()) (()(()))", primitive decomposition yields "(()())" "(())" "(()(()))",

Delete every other part After the outermost bracket in , we get "()()" "()" "()(())" = "()()()()(())".

Example 3:

Input: "()()"

Output: ""

Explanation:

The input string is "()()", and the primitive decomposition is "()" "()".

Delete the outermost brackets in each part Then get "" "" = "".

Tips:

S.length <= 10000
S[i] 为 "(" 或 ")"
S 是一个有效括号字符串
来源：力扣（LeetCode）
class Solution {
    /**
     * @param String $S
     * @return String
     */
    function removeOuterParentheses($S) {
        $strlen = strlen($S);
        $str = &#39;&#39;;
        $sum = 0;
        $star = 1;
        for($i=0;$i<$strlen;$i++){
            if($S[$i] == &#39;(&#39;){
                $sum--;
            }else{
                $sum++;
            }
            if($sum == 0){
                $str.=substr($S,$star,$i-$star);
                $star = $i+2;
            }
        }
        return $str;
    }
}

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