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2. Valid Parentheses

easyAsked at Lyft

Validate that bracket characters in a string close in the right order.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a string s containing just '()[]{}', determine if the input is valid. An input is valid if open brackets are closed by the same type and in the correct order.

Constraints

  • 1 <= s.length <= 10^4
  • s consists of parentheses only

Examples

Example 1

Input
s = "()[]{}"
Output
true

Example 2

Input
s = "(]"
Output
false

Approaches

1. Repeated replace

Strip matched pairs until no change; reject if non-empty.

Time
O(n^2)
Space
O(n)
while (s.includes('()')||s.includes('[]')||s.includes('{}'))
  s = s.replace('()','').replace('[]','').replace('{}','');
return s === '';

Tradeoff:

2. Stack

Push openers, pop on closer and check the match. O(n) one pass.

Time
O(n)
Space
O(n)
function isValid(s) {
  const stack = [];
  const pair = { ')':'(', ']':'[', '}':'{' };
  for (const c of s) {
    if (c in pair) {
      if (stack.pop() !== pair[c]) return false;
    } else {
      stack.push(c);
    }
  }
  return stack.length === 0;
}

Tradeoff:

Lyft-specific tips

Lyft engineers like to relate the stack pattern to balancing nested events in their ride-state machine; mention LIFO ordering explicitly.

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