feat: add valid parentheses

Author: scarowar

problems/0020-valid-parentheses/README.md

@@ -0,0 +1,20 @@
+# Problem 0020: Valid Parentheses
+
+## Link
+https://leetcode.com/problems/valid-parentheses/
+
+## Intuition
+Use a stack to ensure every opening bracket has a matching closing bracket in the correct order.
+
+## Approach
+Iterate through the string, pushing opening brackets onto a stack and popping to match each closing bracket; return false if a mismatch or leftover remains.
+
+## Complexity
+- Time: O(n)
+- Space: O(n)
+
+## Notes
+- Handle cases where the string starts with a closing bracket or ends with unmatched openings.
+- Use a helper function to map each closing bracket to its corresponding opening bracket.
+- Works for all three bracket types: (), {}, [].
+- Returns false immediately on any mismatch or if the stack is not empty at the end.

problems/0020-valid-parentheses/solution.py

@@ -0,0 +1,37 @@
+from collections import deque
+
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        result = deque()
+
+        for c in s:
+            if c in "({[":
+                result.append(c)
+            else:
+                if result:
+                    left = result.pop()
+                    if left != self.getLeft(c):
+                        return False
+                else:
+                    return False
+
+        return not result
+
+    def getLeft(self, c):
+        if c == ")":
+            return "("
+        elif c == "}":
+            return "{"
+        return "["
+
+
+if __name__ == "__main__":
+    s = Solution()
+    print(s.isValid("()"))  # Expected: True
+    print(s.isValid("()[]{}"))  # Expected: True
+    print(s.isValid("(]"))  # Expected: False
+    print(s.isValid("([])"))  # Expected: True
+    print(s.isValid("(()"))  # Expected: False
+    print(s.isValid("("))  # Expected: False
+    print(s.isValid("]"))  # Expected: False

problems/README.md

@@ -3,7 +3,9 @@
 | Number | Title | NeetCode 150 | Link | Folder |
 | ------ | ----- | ------------ | ---- | ------ |
 | 0001 | Two Sum | true | [Link](https://leetcode.com/problems/two-sum/) | [0001-two-sum](0001-two-sum) |
+| 1 | Two Sum | false | [Link](https://leetcode.com/problems/two-sum/) | [1-two-sum](1-two-sum) |
 | 0019 | Remove Nth Node From End Of List | true | [Link](https://leetcode.com/problems/remove-nth-node-from-end-of-list/) | [0019-remove-nth-node-from-end-of-list](0019-remove-nth-node-from-end-of-list) |
+| 0020 | Valid Parentheses | true | [Link](https://leetcode.com/problems/valid-parentheses/) | [0020-valid-parentheses](0020-valid-parentheses) |
 | 0021 | Merge Two Sorted Lists | true | [Link](https://leetcode.com/problems/merge-two-sorted-lists/) | [0021-merge-two-sorted-lists](0021-merge-two-sorted-lists) |
 | 0141 | Linked List Cycle | true | [Link](https://leetcode.com/problems/linked-list-cycle/) | [0141-linked-list-cycle](0141-linked-list-cycle) |
 | 0217 | Contains Duplicate | true | [Link](https://leetcode.com/problems/contains-duplicate/) | [0217-contains-duplicate](0217-contains-duplicate) |