# [LintCode] Segment Tree Build II

### Question

The structure of Segment Tree is a binary tree which each node has two attributes start and 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 build method. 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.

Implement a build method with a given array, so that we can create a corresponding segment tree with every node value represent the corresponding interval max value in the array, return the root of this segment tree.

Similar.

### Code

``````/**
* Definition of SegmentTreeNode:
* public class SegmentTreeNode {
*     public int start, end, max;
*     public SegmentTreeNode left, right;
*     public SegmentTreeNode(int start, int end, int max) {
*         this.start = start;
*         this.end = end;
*         this.max = max
*         this.left = this.right = null;
*     }
* }
*/
public class Solution {
/**
*@param A: a list of integer
*@return: The root of Segment Tree
*/
public SegmentTreeNode build(int[] A) {
if (A == null || A.length == 0) {
return null;
}
return helper(A, 0, A.length - 1);
}

private SegmentTreeNode helper(int[] A, int start, int end) {
if (start > end) {
return null;
}
SegmentTreeNode node = new SegmentTreeNode(start, end);
if (start == end) {
node.max = A[start];
return node;
} else {
node.left = helper(A, start, (start + end) / 2);
node.right = helper(A, (start + end) / 2 + 1, end);
if (node.right != null) {
node.max = Math.max(node.left.max, node.right.max);
} else {
node.max = node.left.max;
}
}
return node;
}
}
``````