LeetCode 56. Merge Intervals

Question

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Example 1:

Input: intervals = [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlaps, merge them into [1,6].

Example 2:

Input: intervals = [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considered overlapping.

Constraints:

• 1 <= intervals.length <= 104
• intervals[i].length == 2
• 0 <= starti <= endi <= 104

Source: https://leetcode.com/problems/merge-intervals/

Solution

For interval problems, sorting the interval array by start is usually a cracking direction.

For sorted interval array, if prev.end < curr.start, there is no overlap between these two intervals Otherwise, they are overlapped.

public int[][] merge(int[][] intervals) {
    // sort by the start in ascending order
    Arrays.sort(intervals, new Comparator<int[]>() {
        @Override
        public int compare(int[] o1, int[] o2) {
            return o1[0] - o2[0];
        }
    });
    Deque<int[]> merged = new LinkedList<>();
    merged.add(intervals[0]);
    for (int[] interval : intervals) {
        int[] last = merged.getLast();
        if (last[1] < interval[0]) {
            merged.addLast(interval);
        } else { // merge
            last[1] = Math.max(last[1], interval[1]);
        }
    }
    int[][] result = new int[merged.size()][];
    merged.toArray(result);
    return result;
}