Given a string containing just parentheses, determine if the input string is valid

Question

The task is taken from leetcode

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

• Open brackets must be closed by the same type of brackets.
• Open brackets must be closed in the correct order.
• Note that an empty string is also considered valid.

Example 1:

Input: "()"

Output: true

Example 2:

Input: "()[]{}"

Output: true

Example 3:

Input: "(]"

Output: false

Example 4:

Input: "([)]"

Output: false

Example 5:

Input: "{[]}"

Output: true

My functional solution

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = s => {
  const par = {
      "(": ")",
      "{": "}",
      "[": "]",
    };
  const rep = [];
  return [...s].every(x => par[x] 
                      ? rep.push(par[x])
                      : rep.pop() === x) && !rep.length;
};

My imperative soltion

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid2 = s => {
  const par = {
      "(": ")",
      "{": "}",
      "[": "]",
    };
  const rep = [];
  for (let i = 0; i < s.length; i++) {
    if (par[s[i]]) {
      rep.push(par[s[i]]);
    } else {
      if (rep.pop() !== s[i]) {
        return false;
      }
    }
  }
  return rep.length === 0;
};

Answer 1

The code looks quite succinct and sufficient. The only suggestions I would offer are minor tweaks:

• for the imperative solution, a for...of loop could be used to eliminate the need to do bookkeeping on the counter variable and use it to index into the string to get each character:

  for(const c of s) {
    if (par[c]) {
      rep.push(par[c]);
    } else {
    if (rep.pop() !== c) {
      return false;
    }
  }
  

• for the functional solution, the callback function could be declared on a previous line, and then the return line can be reduced to a single line:

  const pushOrPopMatch = x => par[x] ? rep.push(par[x]) : rep.pop() === x
  return [...s].every(pushOrPopMatch) && !rep.length;
  

  That line defining pushOrPopMatch() is 73 characters long, which some might argue is too long for a line, given it would be inside a function and indented at least two spaces, so it may not be ideal