SOURCE

var findDisappearedNumbers = function(nums) {
    let len = nums.length, disNums = [], count = 1, i = 0;
    nums = [...new Set(nums)];
    nums.sort((a,b) => {return a - b;});
    while (i < nums.length) {  // 当数组中的元素与count元素相等时才接着往下遍历数组，否则就将count存进返回的结果数组中
        if (nums[i] != count) {
            disNums.push(count);
        } else {
            i++;
        }
        count++;
    }
    while(disNums.length < len - nums.length) {  // 当缺失的是末尾的数字时，通过几个数组长度的比较，判断是否需往结果数组中添加元素
        disNums.push(count);
        count++;
    }
    return disNums;
};
console.log(findDisappearedNumbers([4,3,2,7,7,2,3,1]));