Segment Tree Build II 439

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.

Example

Given [3,2,1,4]. The segment tree will be:

                 [0,  3] (max = 4)
                 /               \
  [0,  1] (max = 3)              [2, 3]  (max = 4)
    /        \                    /                \

[0, 0] (max = 3) [1, 1] (max = 2) [2, 2] (max = 1) [3, 3] (max = 4)

Solution

这题和I没有任何区别，只是多带一个属性max而已。

代码如下：

/**
 * 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) {
        // write your code here
        if(A == null || A.length == 0){
            return null;
        }

        return buildHelper(A, 0, A.length - 1);
    }

    private SegmentTreeNode buildHelper(int[] A, int start, int end){
        if(start == end){
            return new SegmentTreeNode(start, end, A[start]);
        }

        int mid = (start + end) / 2;
        SegmentTreeNode root = new SegmentTreeNode(start, end, 0);
        SegmentTreeNode left = buildHelper(A, start, mid);
        SegmentTreeNode right = buildHelper(A, mid + 1, end);

        root.left = left;
        root.right = right;
        root.max = Math.max(left.max, right.max);

        return root;
    }
}