Next Greater - Implementation

Here's the solution:

function nextGreaterElement(nums1, nums2)
    nextGreater := empty hash map
    stack := empty stack

    for each num in nums2
        while stack is not empty and num > top of stack
            popped := pop from stack
            nextGreater[popped] := num
        push num onto stack

    result := array of size length of nums1
    for i from 0 to length of nums1 - 1
        if nums1[i] in nextGreater
            result[i] := nextGreater[nums1[i]]
        else
            result[i] := -1

    return result

Time: $O(n + m)$. Space: $O(n)$.