// Current solution
for i in 0..mem[b as usize].len() {
    let tmp: i32 = mem[b as usize][i];
    mem[a as usize].push(tmp);
}

// Why cannot I do this
for i in 0..mem[b as usize].len() {
    mem[a as usize].push(mem[b as usize][i]);
}