303. Range Sum Query - Immutable

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https://leetcode.com/problems/range-sum-query-immutable/

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Example:
Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3
Note:
  1. You may assume that the array does not change.
  2. There are many calls to sumRange function.
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