921

Author: ProHiryu

TP/921.md

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+## 921 Minimum Add to Make Parentheses Valid
+
+#### Description
+
+[link](https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/)
+
+---
+
+#### Solution
+
+left records the number of ( we need to add on the left of S.
+right records the number of ) we need to add on the right of S,
+which equals to the number of current opened parentheses.
+
+
+Loop char c in the string S:
+if (c == '('), we increment right,
+if (c == ')'), we decrement right.
+When right is already 0, we increment left
+Return left + right in the end
+
+---
+
+#### Code
+
+O(n)
+O(n)
+
+```python
+def minAddToMakeValid(self, S):
+    left = right = 0
+    for i in S:
+        if right == 0 and i == ')':
+            left += 1
+        else:
+            right += 1 if i == '(' else -1
+    return left + right        
+```