LeetCode: Valid Parentheses Solution

Approach

For each token (character) of s to stack

• if it is a closing token for the peak token of stack, pop
• else push to the stack

The parentheses are valid when stack is empty

Implementation

1/**
2 * @param {string} s
3 * @return {boolean}
4 */
5var isValid = function (s) {
6  function Stack() {
7    const stack = []
8
9    this.push = function (el) {
10      stack.push(el)
11    }
12
13    this.pop = function () {
14      return stack.pop()
15    }
16
17    this.peek = function () {
18      return stack[stack.length - 1]
19    }
20
21    this.isEmpty = function () {
22      return stack.length === 0
23    }
24  }
25
26  const stack = new Stack()
27  const isOpen = token => ["(", "[", "{"].includes(token)
28  const isCloseOf = tokenA => tokenB =>
29    ["()", "[]", "{}"].includes(tokenA + tokenB)
30
31  for (const token of s) {
32    if (isOpen(token)) {
33      stack.push(token)
34    } else if (isCloseOf(stack.peek())(token)) {
35      stack.pop()
36    } else {
37      stack.push(token)
38    }
39  }
40
41  return stack.isEmpty()
42}