Merge Intervalsmedium

fn merge(mut intervals: Vec<Vec<i32>>) -> Vec<Vec<i32>> {
    // TODO: sort by start, then merge overlapping intervals
    vec![]
}

fn main() {
    println!("{:?}", merge(vec![vec![1,3],vec![2,6],vec![8,10],vec![15,18]]));
    println!("{:?}", merge(vec![vec![1,4],vec![4,5]]));
}