Largest Fancy Number At Most N
Call a positive integer *fancy* if every digit in its base-4 representation is either 0 or 1 (so 0, 1, 4, 5, 16, 17, 20, 21, ... are fancy in decimal).
Given an integer $N$ with
\leq N \leq 10^{18}$, return the largest fancy number that is $\leq N$.
Your solution must run in $O(\log N)$ time.
**Examples:**
- $N = 6$: Base-4 digits of 6 are `12`. The largest fancy number $\leq 6$ is 5 (base-4: `11`). Output: `5`
- $N = 1$: Already fancy. Output: `1`
- $N = 100$: Base-4 is `1210`. Largest fancy $\leq 100$ is 85 (base-4: `1111`). Output: `85`
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