Segment Tree Build

Problem

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 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.

Example

Given start=0, end=3. The segment tree will be:

                   [0,  3]
                 /        \
          [0,  1]           [2, 3]
          /     \           /     \
       [0, 0]  [1, 1]     [2, 2]  [3, 3]

Given start=1, end=6. The segment tree will be:

                   [1,  6]
                 /        \
          [1,  3]           [4,  6]
          /     \           /     \
       [1, 2]  [3,3]     [4, 5]   [6,6]
       /    \           /     \
    [1,1]   [2,2]     [4,4]   [5,5]

Clarification

Segment Tree (a.k.a Interval Tree) is an advanced data structure which can support queries like:

  • which of these intervals contain a given point
  • which of these points are in a given interval

See wiki:

  • Segment Tree
  • Interval Tree

Challenge

Solution

Code - Java

/**
 * Definition of SegmentTreeNode:
 * public class SegmentTreeNode {
 * public int start, end;
 * public SegmentTreeNode left, right;
 * public SegmentTreeNode(int start, int end) {
 * this.start = start, this.end = end;
 * this.left = this.right = null;
 * }
 * }
 */
public class Solution {
    /**
     * @param start, end: Denote an segment / interval
     * @return: The root of Segment Tree
     */
    public SegmentTreeNode build(int start, int end) {
        if (start > end) {
            return null;
        }
        SegmentTreeNode node = new SegmentTreeNode(start, end);
        if (start == end) {
            return node;
        }

        node.left = build(start, (start + end) / 2);
        node.right = build((start + end) / 2 + 1, end);

        return node;
    }
}