programming-examples/java/Data_Structures/Java Program to Implement Segment Tree.java

/*This Java program is to Implement Segment tree. In computer science, a segment tree is a tree data structure for storing intervals, or segments. It allows querying which of the stored segments contain a given point. It is, in principle, a static structure; that is, its content cannot be modified once the structure is built. A similar data structure is the interval tree.
A segment tree for a set I of n intervals uses O(n log n) storage and can be built in O(n log n) time. Segment trees support searching for all the intervals that contain a query point in O(log n + k), k being the number of retrieved intervals or segments.*/

public class SegmentTree
{
    private int[] tree;
    private int maxsize;
    private int height;

    private  final int STARTINDEX = 0;
    private  final int ENDINDEX;
    private  final int ROOT = 0;

    public SegmentTree(int size)
    {
        height = (int)(Math.ceil(Math.log(size) /  Math.log(2)));
        maxsize = 2 * (int) Math.pow(2, height) - 1;
        tree = new int[maxsize];
        ENDINDEX = size - 1;
    }

    private int leftchild(int pos)
    {
        return 2 * pos + 1;
    }

    private int rightchild(int pos)
    {
        return 2 * pos + 2;
    }

    private int mid(int start, int end)
    {
        return (start + (end - start) / 2);
    }

    private int getSumUtil(int startIndex, int endIndex, int queryStart, int queryEnd, int current)
    {
        if (queryStart <= startIndex && queryEnd >= endIndex )
            {
                return tree[current];
            }
        if (endIndex < queryStart || startIndex > queryEnd)
            {
                return 0;
            }
        int mid = mid(startIndex, endIndex);
        return  getSumUtil(startIndex, mid, queryStart, queryEnd, leftchild(current))
                + getSumUtil( mid + 1, endIndex, queryStart, queryEnd, rightchild(current));
    }

    public int getSum(int queryStart, int queryEnd)
    {
        if(queryStart < 0 || queryEnd > tree.length)
            {
                return -1;
            }
        return getSumUtil(STARTINDEX, ENDINDEX, queryStart, queryEnd, ROOT);
    }

    private int constructSegmentTreeUtil(int[] elements, int startIndex, int endIndex, int current)
    {
        if (startIndex == endIndex)
            {
                tree[current] = elements[startIndex];
                return tree[current];
            }
        int mid = mid(startIndex, endIndex);
        tree[current] = constructSegmentTreeUtil(elements, startIndex, mid, leftchild(current))
                        + constructSegmentTreeUtil(elements, mid + 1, endIndex, rightchild(current));
        return tree[current];
    }

    public void constructSegmentTree(int[] elements)
    {
        constructSegmentTreeUtil(elements, STARTINDEX, ENDINDEX, ROOT);
    }

    private void updateTreeUtil(int startIndex, int endIndex, int updatePos, int update, int current)
    {
        if ( updatePos < startIndex || updatePos > endIndex)
            {
                return;
            }
        tree[current] = tree[current] + update;
        if (startIndex != endIndex)
            {
                int mid = mid(startIndex, endIndex);
                updateTreeUtil(startIndex, mid, updatePos, update, leftchild(current));
                updateTreeUtil(mid+1, endIndex, updatePos, update, rightchild(current));
            }
    }

    public void update(int update, int updatePos, int[] elements)
    {
        int updatediff = update - elements[updatePos]  ;
        elements[updatePos] = update;
        updateTreeUtil(STARTINDEX, ENDINDEX, updatePos, updatediff, ROOT);
    }

    public static void main(String...arg)
    {
        int[] elements = {1,3,5,7,9,11};
        SegmentTree segmentTree = new SegmentTree(6);
        segmentTree.constructSegmentTree(elements);
        int num = segmentTree.getSum(1, 5);
        System.out.println("the num is " + num);
        segmentTree.update(10, 5,elements);
        num = segmentTree.getSum(1, 5);
        System.out.println("the num is " + num);
    }
}

/*
the sum is 35
the sum is 34