Mismatch - Implementation

Here is the solution:

function findErrorNums(nums)
    n := length of nums
    count := array of size (n + 1) initialized to 0
    for each num in nums
        increment count[num]
    duplicate := -1
    missing := -1
    for i from 1 to n
        if count[i] = 2 then
            duplicate := i
        if count[i] = 0 then
            missing := i
    return [duplicate, missing]

You count frequencies, then scan for the duplicate and missing number. Time: $O(n)$, Space: $O(n)$.