Problem - Range Sum Query Mutable

Given an integer array nums, implement:

• update(index, val): Update nums[index] to val
• sumRange(left, right): Return sum from left to right You solved this with segment trees.

Now solve it with Fenwick Trees. For range sum: $\text{sum}(l, r) = \text{prefix}(r) - \text{prefix}(l-1)$.

Note: Fenwick Trees typically support "add delta" not "set value." To set, compute the delta: $\delta = \text{newVal} - \text{currentVal}$. Constraints: up to $3 \cdot 10^4$ elements.