Add Python solution for Valid Parentheses (Closes #317)

[maitriupadhyay03-cell]

Description

This PR adds a Python solution for the Valid Parentheses problem using a stack-based approach.

Closes #317

Problem

Given a string s containing just the characters (, ), {, }, [ and ], determine if the input string is valid.

A string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 2. Open brackets must be closed in the correct order.
3. 3. Every close bracket has a corresponding open bracket of the same type.

Approach

• Use a stack (list) to track unmatched opening brackets.
• • For each character in the string:
• • If it's an opening bracket (, {, [ → push onto the stack.
• • If it's a closing bracket ), }, ] → check if the top of the stack is the matching opener.

• - If yes → pop the stack.
  

• - If no (or stack is empty) → return `False`.
  

• • At the end, the string is valid only if the stack is empty.

Complexity

	Complexity
Time	O(n) — single pass through the string
Space	O(n) — stack stores at most n/2 brackets

Test Cases

Input	Expected Output
"()"	True
"()[]{}"	True
"(]"	False
"([)]"	False
"{[]}"	True
""	True
"{"	False