add solution: valid-parentheses

Author: dusunax

valid-parentheses/dusunax.py

@@ -0,0 +1,41 @@
+'''
+# 20. Valid Parentheses
+
+use stack data structure to perform as a LIFO
+
+## Time and Space Complexity
+
+```
+TC: O(n)
+SC: O(n)
+```
+
+#### TC is O(n):
+- iterating through the string just once to check if the parentheses are valid. = O(n)
+
+#### SC is O(n):
+- using a stack to store the parentheses. = the worst case is O(n)
+- using a map to store the parentheses. = O(1)
+
+> for space complexity, fixed space is O(1).
+> 👉 parentheses_map is fixed and its size doesn't grow with the input size.
+> 👉 if the map has much larger size? the space complexity is still O(1).
+'''
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = [] # SC: O(n)
+        parentheses_map = { # SC: O(1)
+            "(": ")",
+            "{": "}",
+            "[": "]"
+        }
+
+        for char in s: # TC: O(n) 
+            if char in parentheses_map:
+                stack.append(char)
+            else:
+                if len(stack) == 0 or parentheses_map[stack.pop()] != char:
+                    return False
+        
+        return not stack