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: You may assume that the array does not change. There are many calls to sumRange function.
dp[i]表示从0到i的sum
i到j的sum=dp[j]-dp[i]