The structure of Segment Tree is a binary tree which each node has two attributes
end denote an segment / interval.
start and end are both integers, they should be assigned in following rules:
- The root's start and end is given by
- The left child of node A has
start=A.left, end=(A.left + A.right) / 2.
- The right child of node A has
start=(A.left + A.right) / 2 + 1, end=A.right.
- if start equals to end, there will be no children for this node.
build method with two parameters start and end, so that we can create a corresponding segment tree with every node has the correct start and end value, return the root of this segment tree.