Let's consider about this case:

1
1000000 aaaaaaa...aaaaaaa 1 2 3 4 5 ... 999998 999999 1000000 (string is length 1000000)

In this case your code puts 1000000 strings of length 1000000 in vector v, so your code uses 1000000*1000000 = 1TB. I think this is why the code is MLE.