Next Greater - Algorithm

Two phases:

$1.$ Process $nums2$ with a monotonic decreasing stack. When you pop an element, store its next greater in a hash map.

$2.$ For each element in $nums1$, look up its next greater in the hash map. Return $-1$ if not found. The hash map bridges the two arrays. You compute all next-greater relationships once, then answer queries in $O(1)$. Time: $O(n + m)$. Space: $O(n)$.