Question

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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.

Solution

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;
    }
}