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Valid Parentheses LC #20 Easy Stack · Matching
Problem

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

Example 1
Input: s = "()"
Output: true
Example 2
Input: s = "()[]{}"
Output: true
Example 3
Input: s = "(]"
Output: false
Example 4
Input: s = "{[]}"
Output: true
Constraints: 1 ≤ s.length ≤ 10⁴  |  s consists of parentheses only ()[]{}
Try Examples
Custom:
Approach
Stack: push open brackets, pop & verify on close brackets
If stack is empty or mismatched on close → INVALID. If stack empty at end → VALID
Input String
Stack & Match Check
Stack (top→)
stack
empty
Match Check
— waiting for close bracket —
Variables
Index i
—
char
—
stack size
0
result
—
Step Logic
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Algorithm
1
For each char: if open bracket ( [ { → push onto stack
2
If close bracket & stack is empty → return false (unmatched closer)
3
Pop top of stack; if it doesn't match current closer → return false
4
After loop: return stack.isEmpty() — true iff all opened brackets were closed
Time
O(n)
Space
O(n)
Why Stack Works

Stack mirrors nesting: the most recently opened bracket must be the first to close — LIFO is exactly the nesting order of brackets. Every time we hit a closer, the only valid matching opener is the one on top of the stack.