1. /**
    2.  * @param {string} s
    3.  * @param {string} t
    4.  * @return {character}
    5.  */
    6. var findTheDifference = function(s, t) {
    7.   const hashMap = {}
    9.   for(let i = 0; i < t.length; i++) {
    10.     hashMap[t[i]] ?  hashMap[t[i]] += 1 : hashMap[t[i]] = 1
    11.   }
    13.   for(let i = 0; i < s.length; i++) {
    14.       hashMap[s[i]] -= 1
    15.     if(hashMap[s[i]] === 0) {
    16.       delete hashMap[s[i]]
    17.     }
    18.   }
    20.   return Object.keys(hashMap).join('')
    21. };