class Solution:
def isValid(self, s: str) -> bool:
"""
Determine if the string 's' with brackets is valid.
A string is valid if:
- All open brackets are closed by the same type of brackets.
- Open brackets are closed in the correct order.
- Every closing bracket has a corresponding opening bracket.
Parameters:
s (str): A string comprised of the characters '(', ')', '{', '}', '[' and ']'.
Returns:
bool: True if the string is valid, False otherwise.
"""
# Map closing brackets to their corresponding opening brackets.
bracket_map = {')': '(', '}': '{', ']': '['}
# Initialize an empty stack to keep track of encountered opening brackets.
stack = []
# Process each character in the input string.
for char in s:
if char in bracket_map:
# For each closing bracket, pop the top element from the stack if available.
top_element = stack.pop() if stack else '#'
# If the popped element does not match the corresponding opening bracket, the string is invalid.
if bracket_map[char] != top_element:
return False
else:
# Push opening brackets onto the stack.
stack.append(char)
# If the stack is empty, all brackets were correctly matched.
return not stack
Summary of Techniques and Approaches:
Stack Data Structure: Leveraging a stack to track open brackets is essential when dealing with nested or sequential matching problems. This approach is common for validating parentheses in expressions or ensuring matching pairs in syntax parsing.
Mapping for Quick Lookup: Using a dictionary (or hash map) to map closing brackets to their respective opening brackets allows for efficient comparison during traversal. This technique is applicable to any problem where pair-wise matching or lookup operations are required.
Single Pass Iteration: Traversing the input string only once (O(n) time complexity) leads to an efficient solution. When problems involve verification or transformation of sequences, a single pass strategy can significantly reduce runtime.
Error Handling via Fallback Values: When trying to pop from an empty stack, using a dummy value (like ‘#’ in this case) prevents runtime errors and simplifies conditional checks. This practice can be adapted for handling edge cases in various algorithms.
General Tips: Recognize scenarios with nested structures, paired elements, or ordered dependencies. When these patterns are present, consider utilizing stacks, mapping, or similar data structures to manage state and simplify solution logic.