Q: Given a string, find the most commonly occurring substring

Video walkthrough

const mostCommonSubstring = (str) => {
  const freqCount = {};
  const lcString = str.toLowerCase(); // O(n)

  for (let char of lcString) {  // O(n)
    freqCount[char] = freqCount[char] + 1 || 1;
  }

  let maxCount = 0;
  let mxChar;

  for (let key in freqCount) {  // O(n)
    if (freqCount[key] > maxCount) {
      maxCount = freqCount[key];
      maxChar = key;
    }
  }

  return maxChar;
};

console.log(mostCommonSubstring("DanNy"));

Big O -> 3(n) -> O(n)