I also did it on Java. At first, it only passed up to test 3, due some precision loss, having int k instead of long k. This may be obvious to many here, but I still thought it was interesting to see how such a small detail changes it all. Here is the code (in comments, the code with int, which fails after test 3).

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {
    int N, M, a, b;
    long k; // int k;
    Scanner in = new Scanner(System.in);
    N = in.nextInt();
    M = in.nextInt();
    long[] differenceArray = new long[N+1]; // int[] ...
    for (int i=0; i<M; i++) 
    {
        in.nextLine();
        a = in.nextInt(); 
        b = in.nextInt();
        k = in.nextLong(); // in.nextInt();
        differenceArray[a] += k;
        if (b+1<=N)
            differenceArray [b+1] -= k;
    }

    long max = 0, addedDifference = 0; // int
    for (int i=1; i<=N; i++) 
    {
        addedDifference = addedDifference + differenceArray[i];
        if (max < addedDifference)
            max = addedDifference;
    }
    System.out.println(max);
    in.close();

}

}