How to Fix Valid Parentheses Errors

In this tutorial, you'll learn about How to Fix Valid Parentheses Errors. We cover key concepts, practical examples, and best practices.

Fix valid parentheses errors when stack mismanagement causes incorrect matching or empty stack pop crash.

Quick Fix

Wrong

def is_valid(s):
    st=[]
    for c in s:
        if c in '([{': st.append(c)
        elif c in ')]}': st.pop()
    return len(st)==0

No check if top matches closing bracket. '([)]' returns True.

Right

def is_valid(s):
    p={']':'[','}':'{',')':'('}; st=[]
    for c in s:
        if c in p:
            if not st or st.pop()!=p[c]: return False
        else: st.append(c)
    return not st

'()[]{}' -> True. '([)]' -> False. '(((' -> False. O(n).

Prevention

Map closing to opening. Check stack emptiness before pop. Early return on mismatch.

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FAQ