trainline.py

def f(a, r):
    last = a[0]
    for i in range(len(a)):
        if abs(last - a[i]) > r:
            return False
        last = a[i]
    return True
    
def g(a, r):
    last, j, p = a[0], 0, []
    m = len(a) - 1
    while j < m:
        while j < m and abs(last-a[j]) <= r:
            j+=1
        j-=1
        p.append(a[j])
        last = a[j]
        while j < m and abs(last-a[j]) <= r:
            j+=1
        last=a[j]
    return p

t = [0,1,2,3,6,7,9,11,14,15]
print(f(t,r=3))
print(g(t,r=3))