feat(0020_valid-parentheses): add py3 soln

0020_valid-parentheses/README.md

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+Given a string `s` containing just the characters `'('`, `')'`, `'{'`, `'}'`, `'['` and `']'`, determine if the input string is valid.
+
+An input string is valid if:
+
+1.  Open brackets must be closed by the same type of brackets.
+2.  Open brackets must be closed in the correct order.
+
+**Example 1:**
+
+    Input: s = "()"
+    Output: true
+    
+
+**Example 2:**
+
+    Input: s = "()[]{}"
+    Output: true
+    
+
+**Example 3:**
+
+    Input: s = "(]"
+    Output: false
+    
+
+**Constraints:**
+
+*   `1 <= s.length <= 104`
+*   `s` consists of parentheses only `'()[]{}'`.

0020_valid-parentheses/python3/solution.py

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+from collections import deque
+
+class Solution:
+    bracket_pairing = {
+        '{': '}',
+        '[': ']',
+        '(': ')'
+    }
+    
+    closing_brackets = tuple(bracket_pairing.values())
+    
+    def is_matching_pair(self, opening, closing):
+        return self.bracket_pairing[opening] == closing
+    
+    def isValid(self, s: str) -> bool:
+        stack = deque()
+        
+        for c in s:
+            if c in self.closing_brackets:
+                try:
+                    last_char = stack.pop()
+                except IndexError:
+                    return False
+                
+                if not self.is_matching_pair(last_char, c):
+                    return False
+            else:
+                stack.append(c)
+        
+        return len(stack) == 0