Time: 20 ms (83.79%), Space: 13.5 MB (81.46%) - LeetHub

20-valid-parentheses/20-valid-parentheses.py

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+class Solution(object):
+    def isValid(self, s):
+        """
+        :type s: str
+        :rtype: bool
+        """
+        openBrackets = "({["
+        closedBrackets = ")}]"
+        openStack = ""
+        
+        for ch in s:
+            if ch in openBrackets:
+                # Add open brackets to the stack
+                openStack += ch
+            else: # elif ch in closedBrackets: (Contrant given: s only contains brackets)
+                # Invalid if no open brackets
+                if len(openStack) == 0:
+                    return False
+                
+                # Check that the last bracket on stack is same type as close
+                if openStack[-1] == openBrackets[closedBrackets.index(ch)]:
+                    openStack = openStack[:-1]
+                else:
+                    return False
+        
+        # Only true if no outstanding open brackets
+        return openStack == ""