remove_outmost_parentheses.go

/*
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 "" + "" = "".


Note:

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



*/

// time: 2020-05-04

package removeoutmostparentheses

// time complexity: O(n), where n is length of S.
// space complexity: O(1)
func removeOuterParentheses(S string) string {
	// S is valid parentheses string, so, s is empty "" or starts with "("
	if len(S) == 0 {
		return S
	}

	var stackLength, left int

	var ret string

	for i := 0; i < len(S); i++ {
		if stackLength == 0 {
			left = i
		}
		if S[i] == '(' {
			stackLength++
		} else {
			stackLength--
		}
		if stackLength == 0 {
			ret += S[left+1 : i]
		}
	}
	return ret
}