Merge Overlapping Intervals

Ideone.java:

import java.util.*;

public class Ideone
{   
    public static void main (String[] args) throws java.lang.Exception
    {
        ArrayList<Interval> x = new ArrayList<>();

        x.add(new Interval(1, 3));
        x.add(new Interval(2, 6));
        x.add(new Interval(8, 10));
        x.add(new Interval(15, 18));
        x.add(new Interval(17, 20));

        x = merge(x);

        for(Interval i : x)
        {
            System.out.println(i.getStart() + " " + i.getEnd());
        }
    }

    public static ArrayList<Interval> merge(ArrayList<Interval> intervals) {

        if(intervals.size() == 0 || intervals.size() == 1)
            return intervals;

        Collections.sort(intervals, new IntervalComparator());

        Interval first = intervals.get(0);
        int start = first.getStart();
        int end = first.getEnd();

        ArrayList<Interval> result = new ArrayList<Interval>();

        for (int i = 1; i < intervals.size(); i++) {
            Interval current = intervals.get(i);
            if (current.getStart() <= end) {
                end = Math.max(current.getEnd(), end);
            } else {
                result.add(new Interval(start, end));
                start = current.getStart();
                end = current.getEnd();
            }
        }

        result.add(new Interval(start, end));
        return result;
    }
}

class Interval 
{
    private int start;
    private int end;

    Interval() {
        start = 0;
        end = 0;
    }

    Interval(int s, int e) 
    {
        start = s;
        end = e;
    }

    public int getStart() {
        return start;
    }

    public int getEnd() {
        return end;
    }
}

class IntervalComparator implements Comparator<Interval>
{
    public int compare(Interval i1, Interval i2)
    {
        return i1.getStart() - i2.getStart();
    }
}

• main method is inside the public class Ideone
• Interval and IntervalComparator are just inner classes

Output:

1 6
8 10
15 20

My implementation using BST. O(logn) to merge: Inorder traversal gives you updated intervals.

private static Interval add(Interval root, Interval x){
    if(root == null)
        return x;
    if(root.start >= x.start){
        root.left = add(root.left,x);
        if(mergeLeft(root,root.left)){
            root.left = null;
        }               
    }
    else{
        root.right = add(root.right,x);
        if(mergeRight(root,root.right)){
            root.right = null;
        }
    }

    return root;
}

private static boolean mergeLeft(Interval root,Interval left){
    if(left.end < root.start)
        return false;

    else{
        root.start = Math.min(root.start, left.start);
        root.end = Math.max(root.end, left.end);
        return true;
    }
}

private static boolean mergeRight(Interval root,Interval right){
    if(right.start > root.end)
        return false;
    else{
        root.start = Math.min(root.start, right.start);
        root.end = Math.max(root.end, right.end);
        return true;
    }
}

Here is your refined code:

import java.util.*;
import java.lang.*;
import java.io.*;

class Interval {
    int start;
    int end;

    Interval() {
        start = 0;
        end = 0;
    }

    Interval(int s, int e) {
        start = s;
        end = e;
    }
}

public class Ideone {
    public static void main(String[] args) throws java.lang.Exception {
        ArrayList<Interval> x = new ArrayList<Interval>();
        Interval i1 = new Interval(1, 3);
        Interval i2 = new Interval(2, 6);
        Interval i3 = new Interval(8, 10);
        Interval i4 = new Interval(15, 18);

        x.add(i1);
        x.add(i2);
        x.add(i3);
        x.add(i4);

        ArrayList<Interval> r = merge(x);

        for (Interval i : r) {
            System.out.println(i.start + " " + i.end);
        }

    }

    public static ArrayList<Interval> merge(ArrayList<Interval> intervals) {

        if (intervals.size() == 0)
            return intervals;
        if (intervals.size() == 1)
            return intervals;

        Collections.sort(intervals, new IntervalComparator());

        Interval first = intervals.get(0);
        int start = first.start;
        int end = first.end;

        ArrayList<Interval> result = new ArrayList<Interval>();

        for (int i = 1; i < intervals.size(); i++) {
            Interval current = intervals.get(i);
            if (current.start <= end) {
                end = Math.max(current.end, end);
            } else {
                result.add(new Interval(start, end));
                start = current.start;
                end = current.end;
            }

        }

        result.add(new Interval(start, end));

        return result;

    }
}

class IntervalComparator implements Comparator {
    public int compare(Object o1, Object o2) {
        Interval i1 = (Interval) o1;
        Interval i2 = (Interval) o2;
        return i1.start - i2.start;
    }
}

And name this java file "Ideone.java"