MergeIntervals.java

package com.freetymekiyan.algorithms.level.hard;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * Given a collection of intervals, merge all overlapping intervals.
 * <p>
 * For example,
 * Given [1,3],[2,6],[8,10],[15,18],
 * return [1,6],[8,10],[15,18].
 * <p>
 * Company Tags: LinkedIn, Google, Facebook, Twitter, Microsoft, Bloomberg, Yelp
 * Tags: Array, Sort
 * Similar Problems: (H) Insert Interval, (E) Meeting Rooms, (M) Meeting Rooms II
 */
public class MergeIntervals {


    /**
     * Sort. O(nlogn) Time.
     * Sort the intervals by start time, ascending.
     * Use a pointer, prev, for previous merged interval.
     * For each of the intervals:
     * | If prev == null, merged is empty, add directly and update prev.
     * | If prev.end < i.start, no overlap, add directly and update prev.
     * | Else if prev.end >= i.start, there is overlap.
     * |   We already know i.start >= prev.start because of sorting. No need to update start.
     * |   Only update end is enough.
     * |   If prev.end >= i.end, no need to update end.
     * |   If prev.end < i.end, update end to i.end.
     */
    public List<Interval> merge(List<Interval> intervals) {
        if (intervals == null || intervals.size() <= 1) {
            return intervals;
        }
        List<Interval> merged = new ArrayList<>();
        Collections.sort(intervals, (i1, i2) -> Integer.compare(i1.start, i2.start));
        Interval prev = null; // A pointer to the last interval in merged list.
        for (Interval i : intervals) {
            if (prev == null || prev.end < i.start) { // Empty list or no overlap.
                merged.add(i);
                prev = i; // Update previous pointer.
            } else if (prev.end < i.end) { // Overlap and the end of current interval is larger.
                prev.end = i.end; // Update previous end to merge.
            }
        }
        return merged;
    }

    /**
     * Interval class provided by leetcode.
     */
    public class Interval {

        int start;
        int end;

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

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