Range Frequency Solution

For this problem, the hash map approach is sufficient and simpler.

However, if you needed to extend to queries like "count values in range $[a, b]$" or "find $k$-th smallest in range," the wavelet tree becomes necessary.

Implementation notes:

• Use a dictionary mapping values to sorted lists of indices
• Binary search to count indices in $[l, r]$
• Handle missing values gracefully

Time: $O(n)$ preprocessing, $O(\log n)$ per query. Space: $O(n)$.