SOURCE

/**
 * @param {number[]} nums
 * @return {number[]}
 */
var findDisappearedNumbers = function(nums) {
    for(var i = 0; i < nums.length; )
        {
            var index = nums[i];
            if(nums[index - 1] != index)
                {
                    nums[i] = nums[index - 1];
                    nums[index - 1] = index;
                }
            else
                i++;
        }
    var res = [];
    for(var i = 0; i < nums.length; i++)
        {
            if(nums[i] != i + 1)
                res.push(i + 1);
        }
    return res;
};
console.log(findDisappearedNumbers([4,3,2,7,8,2,3,1]));