AoC15.py

with open("input.txt") as f:
    data = [int(i) for i in f.read().strip().split(',')]

print(data)

last_num = 0 
history = {}
for i, num in enumerate(data):
    if i != 0:
        history[last_num] = i 
    last_num = num 

for i in range(len(data), 30000000):
    if last_num not in history:
        history[last_num] = i 
        last_num = 0 
    else:
        val = i - history[last_num]
        history[last_num] = i 
        last_num = val 

print(last_num)