Segment Tree | Set 1 (Sum of given range) Let us consider the following problem to understand Segment Trees. We have an array arr[0 . . . n-1]. We should be able to 1 Find the sum of elements from index l to r where 0 <= l <= r <= n-1 2 Change value of a specified element of the array arr[i] = x where 0 <= i <= n-1.