Link: https://leetcode.com/problems/lexicographical-numbers/
Solution:
Topics: math
Intuition
This is a pretty tough problem, probably should not be a medium but overall its not that bad if you can recognize the pattern. The difficulty comes from the fact that it must be solved in o(n) time and o(1) space. I think the problem remains a medium because o(nlogn) solutions are also accepted by the judge.
Consider this:
n = 11
lexographically ordered:
[1,10,11,2,3,4,5,6,7,8,9]
We notice that 10 immediately follows 1 because a trailing 0 adds the lowest amount of lexicographical value. If n=100 then the the three first elements would be [1, 10, 100, ...].
Hopefully the pattern is clear, we must do curr *= 10 if curr*10 <= n…otherwise we increment by one! Half the problem is solved.
If we look at the above example, we see that we must transform 11 into 2. This is kind of tricky, but we see that if we increment 11 we get 12 which is greater than n, and therefore not part of the list. Its easy to think that we can just take away 10 and get left with a 2, but if we consider the case where n=13, the next lexicographical element after 13 is still 2! So what’s the pattern here?
The key insight is in realizing that after 11, we must jump to 20 by rounding up to the nearest 10 and then integer divide by 10 to get our 2!
Another case to consider is if n=200…in which case at some point in our list we should have [..., 199, 2, ...]. If we jump to 200 from 199 and then integer divide once, we are left with 20…so we must integer divide by 10 WHILE the number is divisible by 10!
Implementation
def lexico_nums(n):
res = []
curr = 1
for _ in range(n):
res.append(curr)
if curr * 10 <= n:
curr *= 10
else:
curr += 1
if curr > n:
curr = ((curr + 9) // 10) * 10
# ^round up to nearest 10. The *10 at the end is spurious
# because it gets removed by the integer division that follows.
# I'm going to keep it because its a full "rounding up" and
# and more in line with the intuition.
while curr % 10 == 0: #remove trailing zeros
curr //= 10
return res
#time: o(n)
#memory: o(1)Mnemonic
*= 10, += 1, //= 10
times ten, plus one, integer divide 10