Stacks and Queues

Balanced Parentheses

Walk a string of bracket characters. Push every opening bracket. On a closing bracket, pop and verify it matches; mismatch or empty-stack pop means unbalanced. At end of string, stack must be empty.

Algorithm

Canonical input "({[]})" (balanced) finishes with the stack empty and the result true.

Basic Implementation

basic.js 
const text = "({[]})";
const pairs = { ")": "(", "]": "[", "}": "{" };
const stack = [];
let balanced = true;
for (let i = 0; i < text.length; i++) {
    const ch = text[i];
    if (ch === "(" || ch === "[" || ch === "{") {
        stack.push(ch);
    } else {
        if (stack.length === 0 || stack[stack.length - 1] !== pairs[ch]) {
            balanced = false;
            break;
        }
        stack.pop();
    }
}
if (stack.length !== 0) {
    balanced = false;
}
console.log(balanced);

Complexity

  • Time: O(n)
  • Space: O(n) worst case

Implementation notes

  • JavaScript: a plain array doubles as a stack via push and pop; avoid any external stack library.
  • The replay highlights the current character, shows the stack updating each frame, and surfaces the final balanced/unbalanced verdict.
stack push/pop Use a plain array (`push` / `pop`) as the stack.
matching map A small object literal mapping each closing bracket to its expected opener keeps the lesson compact.