added changes to common_pref and added new solution

Author: SanjarH

common_pref.py

@@ -9,8 +9,9 @@ def longestCommonPrefix(strs):
         return ""
     output = "" # empty output
     shortest_str = min(strs, key=len) # get the shortest string in the list 
-    for i in range(len(shortest_str)): # check each characters of the shortest string 
-        if all(s[i] == shortest_str[i] for s in strs): # check if every string have the same character at index i 
+    for i in range(len(shortest_str)): # check each characters of the shortest string
+        # check if every string have the same character at index i 
+        if all(s[i] == shortest_str[i] for s in strs): 
             output += shortest_str[i]     # add character to output 
         else:
             break               # otherwise return empty output

parentheses.py

@@ -0,0 +1,33 @@
+# Learn Python together
+"""Write a python program to check parentheses is valid or not"""
+
+# Solution 1 using for loop, append and pop methods
+def isValid(strs):
+    open_parenth = ["(", "[", "{"]
+    close_parenth = [')', ']', "}" ]
+    stack = []
+
+    for char in strs:
+        if char in open_parenth:
+            stack.append(char)
+        elif char in close_parenth:
+        # check if empty stack or if the last item in the stack matches the current char 
+            if not stack or open_parenth.index(stack[-1]) != close_parenth.index(char):  
+                return False
+            stack.pop() # pop method used for removing the last char
+    return not stack  
+# check       
+print(isValid("()[]{")) # False
+print(isValid("()[]{}")) # True
+
+# Solution 2 using while loop, any and replace methods
+def isValid2(strs):
+    list_parentheses = ["[]", "()", "{}"]
+    # iteration for any chars in strs and list and check if they are matched
+    while any(char in strs for char in list_parentheses):
+        for par in list_parentheses:
+            strs = strs.replace(par, "") # if are matched remove last char
+    return not strs
+# check     
+print(isValid2("()[]{")) # False
+print(isValid2("()[]{}")) # True