validString.js

/**
Sherlock considers a string to be valid if all characters of the string appear the same number of times. It is also valid if he can remove just 1 character at 1 index in the string, and the remaining characters will occur the same number of times. Given a string s, determine if it is valid. If so, return YES, otherwise return NO.

Output Format

Print YES if string

is valid, otherwise, print NO.

Sample Input 0

aabbcd

Sample Output 0

NO

 */

const isValid = (s) => {
  let counter = {};
  let freq = {};

  // count of occurrences of characters
  // for example aaabbcc => {a: 3, b: 2, c: 2}
  Array.from(s).forEach((char) => {
    counter[char] = counter[char] || 0;
    counter[char]++;
  });

  // count the frequency of those occurence counts,
  // for example {a: 3, b: 2, c: 2} => {3: 1, 2: 2}
  Object.keys(counter).forEach((k) => {
    freq[counter[k]] = freq[counter[k]] || 0;
    freq[counter[k]]++;
  });

  // all our frequencies. e.g. [3, 2]
  let freqArr = Object.keys(freq).map(Number);

  // Simplest case: if there is only one frequency, it's valid
  if (freqArr.length === 1) {
    return "YES";
  }

  // If not a single frequency string, check next simplest case

  // there are exactly two frequencies
  // e.g. [3, 2]
  let twoFrequencies = freqArr.length === 2;
  let [a, b] = freqArr;

  // frequency occurs only once
  let oneOccurence = freq[a] === 1 || freq[b] === 1;

  // the difference is a singleton character
  // e.g. abbcc
  let singleton = (freq[a] === 1 && a === 1) || (freq[b] === 1 && b === 1);

  // differing frequency is exactly 1 more than others
  // e.g. true if aaabbcc, false if abbcc
  let diffOfOne = (freq[a] === 1 ? a - b : b - a) === 1;

  // if single character or frequencies differ by one
  // and there are exactly two frequencies
  // and there is only one occurence of the singleton or differing frequency
  if ((singleton || diffOfOne) && twoFrequencies && oneOccurence) {
    return "YES";
  }

  return "NO";
};

console.log(isValid("aabbcd")); // NO
// console.log(isValid("aabbccddeefghi")); // NO
// console.log(isValid("abcdefghhgfedecba")); // YES