Find All Numbers Disappeared In Array

Working through this problem, I noticed it is very similar to the previous problem I encountered (268). However, in this problem you have to return an array of missing numbers instead of just one, which complicates it slightly. I got to this point rather quickly utilizign a hash map instead of sorting:

func findDisappearedNumbers(nums []int) []int {
    output := []int{}
    hashMap := make(map[int]int)

    for _,num := range nums {
        hashMap[num]++
    }

    for i := 1; i < len(nums); i++ {
        if _,exists := hashMap[i]; !exists {
            output = append(output, i)
        }
    }

    return output
}

Afterwards I thought, I need a way to keep track of whether or not the last value was added to the output list or not, because in some instances it is, and in others it isn't. My basic solution was to keep a running tally everytime a number occured more than once. For example with the input [1,1], the corresponding hashMap is map[1:2]. And this is where the following check comes into play.

if hashMap[i] > 1 {
            total = total + hashMap[i] - 1
        }

Here I calculate the number of missing numbers there are in total. In the example previously mentioned, the total would become 2 - 1 = 1. Meaning there is one number missing from the output. Then I check at the very end if there is exactly one missing number, if there is, it is the len(nums), so it can safely be appended. I have no doubt there is a better implementation and I will revisit this in the near future and likely find a much simplier solution.

func findDisappearedNumbers(nums []int) []int {
    output := []int{}
    hashMap := make(map[int]int)

    for _,num := range nums {
        hashMap[num]++
    }

    total := 0
    for i := 1; i < len(nums); i++ {
        if _, exists := hashMap[i]; !exists {
            output = append(output, i)
        }
        if hashMap[i] > 1 {
            total = total + hashMap[i] - 1
        }
    }

    missingNumber := total - len(output)

    if missingNumber == 1 {
        output = append(output, len(nums))
    }
    return output
}