The Merge Template in Python, Java and TypeScript

def merge(intervals: list[list[int]]) -> list[list[int]]:
    ordered = sorted(intervals, key=lambda interval: interval[0])
    merged: list[list[int]] = []
    for start, end in ordered:
        if merged and start <= merged[-1][1]:
            merged[-1][1] = max(merged[-1][1], end)
        else:
            merged.append([start, end])
    return merged


intervals = [[4, 7], [1, 3], [8, 11], [2, 5], [12, 13]]
print("input: ", intervals)
print("merged:", merge(intervals))

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class Merge {
    static List<int[]> merge(int[][] intervals) {
        int[][] ordered = intervals.clone();
        Arrays.sort(ordered, Comparator.comparingInt(interval -> interval[0]));
        List<int[]> merged = new ArrayList<>();
        for (int[] interval : ordered) {
            int[] last = merged.isEmpty() ? null : merged.get(merged.size() - 1);
            if (last != null && interval[0] <= last[1]) {
                last[1] = Math.max(last[1], interval[1]);
            } else {
                merged.add(new int[] {interval[0], interval[1]});
            }
        }
        return merged;
    }

    static String show(List<int[]> intervals) {
        StringBuilder text = new StringBuilder();
        for (int[] interval : intervals) {
            if (text.length() > 0) {
                text.append(", ");
            }
            text.append(Arrays.toString(interval));
        }
        return "[" + text + "]";
    }

    public static void main(String[] args) {
        int[][] intervals = {{4, 7}, {1, 3}, {8, 11}, {2, 5}, {12, 13}};
        System.out.println("input:  " + show(Arrays.asList(intervals)));
        System.out.println("merged: " + show(merge(intervals)));
    }
}

type Interval = [number, number];

function merge(intervals: Interval[]): Interval[] {
  const ordered = [...intervals].sort((x, y) => x[0] - y[0]);
  const merged: Interval[] = [];
  for (const [start, end] of ordered) {
    const last = merged[merged.length - 1];
    if (last !== undefined && start <= last[1]) {
      last[1] = Math.max(last[1], end);
    } else {
      merged.push([start, end]);
    }
  }
  return merged;
}

const show = (intervals: Interval[]): string =>
  "[" + intervals.map(([start, end]) => `[${start}, ${end}]`).join(", ") + "]";

const intervals: Interval[] = [[4, 7], [1, 3], [8, 11], [2, 5], [12, 13]];
console.log("input:  " + show(intervals));
console.log("merged: " + show(merge(intervals)));