Daily Leetcode 1647 - 28/06/2022

Problem Summary

Today’s problem is 1647: Minimum Deletions to Make Character Frequencies Unique. Our goal is to write n, an integer as the minimum number of deci-binary numbbers, e.g 32 = 10+11+11. There are multiple ways to write 32 as the sum of deci-binary numbers but we’re looking at the smallest.

My thought process

This one is tricky, but has a very short solution. I simply tabulated values, noticing that single digit numbers are their own answer. Then, if I took a number, say 17, I’d need 1+1+1+1+1+1+11 which is 7 terms. Then, I looked at 39 = 11+11+11+(1+1+1+1+1+1) which is 9 terms. There is a pattern of the result being the largest digit in n. The idea is that we’re going to need k terms to build up that specific digit, and is the limiting factor.

The code

py
class Solution:
def minDeletions(self, s: str) -> int:
count = 0
seen = set()
freqs = [0 for _ in range(26)]
'''
Count frequencies, decrement uniques till we have a slot
'''
for char in s:
freqs[ord(char)-ord('a')] += 1
for idx in range(26):
while freqs[idx] and freqs[idx] in seen:
freqs[idx] -= 1
count += 1
seen.add(freqs[idx])
return count

Pitfalls, and where I stumbled

It took a while to realise the pattern, and the independence of digits to the left/right of the largest digit. I also had considered a recursive solution but this is overkill, and would be too slow. There are similar such number related problems where you’ll need to look for a pattern to simplify your logic to $O(n)$. The idea: if you observe a pattern, run a few examples by hand and test it!