P961
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Removing Digits

ℹ️Published on Sunday, 21st September 2025, 05:00 am; Solved by 728;
Difficulty level: 4

This game starts with a positive integer. Two players take turns to remove a single digit from that integer. After the digit is removed any resulting leading zeros are removed.

For example, removing a digit from $105$ results in either $5$, $10$ or $15$.

The winner is the person who removes the last nonzero digit.

Define $W(N)$ to be how many positive integers less than $N$ for which the first player can guarantee a win given optimal play. You are given $W(100) = 18$ and $W(10^4) = 1656$.

Find $W(10^{18})$.



Soluzione

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