1. Open-bracket stack
Push opens; the most recent must match the next close.
const stack = []Typorithm · easy
Valid Parentheses
valid-parenthesesstacks-queuesstackmatchinglifo
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Valid Parentheses
Stacks & Queues
Push openers on a stack; each closer must match the most recent opener, and the stack must end empty.
Brackets nest, so the last opened must be the first closed — classic LIFO.
A stack holds open brackets; each closer pops and checks against its required opener.
A mismatch or a leftover opener at the end means the string is invalid.
01
Push openers, pop on closers.
02
Top opener must match the closer.
03
End with an empty stack.
function isValid(s) {
const stack = [];
const pairs = { ")": "(", "]": "[", "}": "{" };
for (const ch of s) {
if (ch === "(" || ch === "[" || ch === "{") {
stack.push(ch);
} else if (stack.pop() !== pairs[ch]) {
return false;
}
}
return stack.length === 0;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Close → open map
Map each closing bracket to its required opener.
const pairs = { ")": "(", "]": "[", "}": "{" }3. Push opener
Remember every opening bracket on the stack.
stack.push(ch)4. Match the close
The top opener must match the current closer.
stack.pop() !== pairs[ch]5. Fully closed
An empty stack means every bracket was matched.
return stack.length === 0A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.