Valid Parentheses - ELI5 Park

👶 The 5-Year-Old Summary

You have a bunch of brackets like ()[]{}. Does every opening bracket have its matching closing bracket that properly nests inside? Like properly stacking and unstacking plates!

🎮 Interactive Visualizer

Click "Step" to see the stack in action!

Input:

Stack (plates):

🧠 The Brain Hack

When brackets need to "match" or "nest," think STACK! Opening bracket → push to stack. Closing bracket → pop and check if it matches!

💻 The Clean Solution

def is_valid(s):
    # A stack to hold opening brackets
    stack = []
    # Matching pairs
    mapping = {')': '(', '}': '{', ']': '['}
    
    for char in s:
        # If it's a closing bracket
        if char in mapping:
            # Pop from stack (or use placeholder if empty)
            top = stack.pop() if stack else '#'
            
            # Does it match?
            if mapping[char] != top:
                return False
        else:
            # It's an opening bracket - push to stack
            stack.append(char)
    
    # Valid if stack is empty (all matched)
    return len(stack) == 0

# Try it!
print(is_valid("([])"))   # True
print(is_valid("([)]"))   # False

Time: O(n) - one pass through the string

Space: O(n) - worst case, all opening brackets