One of the more popular ways to solve this, spoiler in case you want to figure it out:
Probabilistic Method
Remember that there are 4 billion numbers, in a 32-bit address space, which is 4294967296 in size, meaning that 294967296 are available, just need to find one. So if you, as an example, just select 100 unique numbers, then parse through the input once weeding them out for what you find, then you have a greater than 99.9% chance of finding a valid number just in the first pass, and if it doesn’t work then you can repeat it again and again until it does, probabilistically it will work quickly, and even more quickly if you select even more numbers to test with, 10 megs worth of numbers (2500000 of them) means that even in the worst case you loop over the input again and again 1599 times, which is still not ‘too’ bad even if not great, and that would only happen if you didn’t select any of the 294967296 available numbers until the very end.
There are other ways still!