1003-check-if-word-is-valid-after-substitutions.js

/**
 * 1003. Check If Word Is Valid After Substitutions
 * https://leetcode.com/problems/check-if-word-is-valid-after-substitutions/
 * Difficulty: Medium
 *
 * Given a string s, determine if it is valid.
 *
 * A string s is valid if, starting with an empty string t = "", you can transform t into s after
 * performing the following operation any number of times:
 * - Insert string "abc" into any position in t. More formally, t becomes tleft + "abc" + tright,
 *   where t == tleft + tright. Note that tleft and tright may be empty.
 *
 * Return true if s is a valid string, otherwise, return false.
 */

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function(s) {
  const stack = [];

  for (const char of s) {
    if (char === 'c') {
      if (stack.length < 2 || stack.pop() !== 'b' || stack.pop() !== 'a') {
        return false;
      }
    } else {
      stack.push(char);
    }
  }

  return stack.length === 0;
};