From ba5483bd65b7b1a9d02c24f1087b3e08e24dac7f Mon Sep 17 00:00:00 2001
From: Yuri <YuriSpiridonov@users.noreply.github.com>
Date: Sun, 19 Jul 2020 14:51:22 +0300
Subject: [PATCH] Python solution for problem # 20

new solution and readme update
---
 Python/20_ValidParentheses.py | 31 +++++++++++++++++++++++++++++++
 README.md                     | 12 ++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 Python/20_ValidParentheses.py

diff --git a/Python/20_ValidParentheses.py b/Python/20_ValidParentheses.py
new file mode 100644
index 00000000..17c2d336
--- /dev/null
+++ b/Python/20_ValidParentheses.py
@@ -0,0 +1,31 @@
+# -*- coding: utf-8 -*-
+#Difficulty: Easy
+#Runtime: 82.23% 
+#Memory Usage: 64.83%
+
+"""
+Algorithm:
+    1. Check a bracket (b) in the string (s), if it contains in parentheses 
+    dictionary then we add to stack it's opposite bracket.
+    2. If bracket not in parentheses we check does it mach with last bracket in 
+    the stack, also we check if stack exists while iterating through the string. 
+    If one of this conditions is not True - return False immediately.
+    3. After we checked whole string we need to check is stack empty or not and  
+    return result.
+    4. Result will be true if stack is empty it means all parentheses are valid 
+    and closed in the correct order.
+
+Time Complexity =  O(n)
+Space Complexity = O(n)
+"""
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        parentheses = {'(':')', '{':'}', '[':']'}
+        stack = []
+        for b in s: # take bracket 'b' from string 's'
+            if b in parentheses: # if bracket in parentheses
+                stack.append(parentheses[b]) # append it's opposite to stack
+            elif not stack or stack.pop() != b: # if not stack or bracket not 
+                return False                    # equal last bracket in stack
+        return not stack # if stack still exists -> False else True
\ No newline at end of file
diff --git a/README.md b/README.md
index 7648412c..86550ff5 100644
--- a/README.md
+++ b/README.md
@@ -100,6 +100,18 @@
 </div>
 <br/>
 
+## Stack
+|  #  | Title           |  Solution       |  Time           | Space           | Difficulty    | Tag          | Note| 
+|-----|---------------- | --------------- | --------------- | --------------- | ------------- |--------------|-----|
+|020|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)|[Python](./Python/20_ValidParentheses.py)|_O(n)_|_O(n)_|Easy|Stack||
+
+<br/>
+<div align="right">
+    <b><a href="#algorithms">⬆️ Back to Top</a></b>
+</div>
+<br/>
+
+
 ## Graph
 |  #  | Title           |  Solution       |  Time           | Space           | Difficulty    | Tag          | Note| 
 |-----|---------------- | --------------- | --------------- | --------------- | ------------- |--------------|-----|