1021. Remove Outermost Parentheses

README.md

@@ -63,6 +63,7 @@ continually updating 😃.
 * [345. Reverse Vowels of a String](src/0345_reverse_vowels_of_a_string/reverse_vowels.go)   *`string;`*  *`double index`*
 * [438. Find All Anagrams in a String](src/0438_all_anagrams_in_a_string/all_anagrams_in_a_string.go)   *`sliding window`*
 * [557. Reverse Words in a String III](src/0557_reverse_words_in_a_string_3/reverse_words_in_a_string_3.go)
+* [1021. Remove Outermost Parentheses](src/1021_Remove_Outermost_Parentheses/remove_outmost_parentheses.go)   *`stack`*
 
 ### Linked List
 * [2. Add Two Numbers](src/0002_add_two_numbers/add_two_numbers.go)   *`recursion;`*  *`math`*

src/1021_Remove_Outermost_Parentheses/remove_outmost_parentheses.go

@@ -0,0 +1,77 @@
+/*
+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
+}

src/1021_Remove_Outermost_Parentheses/remove_outmost_parentheses_test.go

@@ -0,0 +1,20 @@
+package removeoutmostparentheses
+
+import (
+	"testing"
+)
+
+func TestRemoveOuterParentheses(t *testing.T) {
+	testCases := []map[string]string{
+		{"case": "(()())(())", "expected": "()()()"},
+		{"case": "(()())(())(()(()))", "expected": "()()()()(())"},
+		{"case": "", "expected": ""},
+		{"case": "()()", "expected": ""},
+	}
+
+	for _, testCase := range testCases {
+		if testCase["expected"] != removeOuterParentheses(testCase["case"]) {
+			t.Errorf("hello")
+		}
+	}
+}

src/README.md

@@ -114,4 +114,5 @@
 |0728|[Self Dividing Numbers](./0728_self_dividing_numbers/self_dividing_numbers.go)|Easy||
 |0735|[735. Asteroid Collision](0735_asteroid_collision/ac.go)|Medium|*`stack`*|
 |0747|[Largest Number At Least Twice of Others](./0747_largest_number_at_least_twice_of_others/largest_number_at_least_twice_of_others.go)|Easy||
-|0872|[872. Leaf-Similar Trees](0872_leaf_similar_trees/leaf_similar_trees.go)|Easy|*`binary tree`*|
+|0872|[872. Leaf-Similar Trees](0872_leaf_similar_trees/leaf_similar_trees.go)|Easy|*`binary tree`*|
+|1021|[1021. Remove Outermost Parentheses](1021_Remove_Outermost_Parentheses/remove_outmost_parentheses.go)|Easy|*`stack`*|