let arr = [11, 11, 2, 2, 5, 5, 5, 5, 3];
//創建一個map，把每個數字和其個數相對應
let countObj = {};
for (i = 0; i <= arr.length - 1; i++) {
let v = arr[i];
if (countObj[v]) {
countObj[v]++;
} else {
countObj[v] = 1;
}
}
// console.log(countObj); //{ '2': 2, '3': 1, '5': 4, '11': 2 }
for (let key in countObj) {
if (countObj[key] === 1) {//判斷在對象中出現為1次的是哪個屬性
console.log(key);//3...
}
}