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2. Valid Parentheses

easyAsked at Byju's

Determine whether a string of brackets is correctly balanced and nested.

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

Problem

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

Constraints

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

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Repeated replace

Repeatedly remove matched pairs until stable.

Time
O(n^2)
Space
O(n)
while (s.includes('()')||s.includes('[]')||s.includes('{}'))
  s = s.replace('()','').replace('[]','').replace('{}','');
return s.length === 0;

Tradeoff:

2. Stack match

Push openers onto a stack; pop and verify on closers. Empty stack at the end signals validity.

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

Tradeoff:

Byju's-specific tips

Byju's loves stack questions because they map cleanly to syllabus content, so be ready to teach the stack invariant out loud.

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