788. Rotated Digits

X is a good number if after rotating each digit individually by 180 degrees, we get a valid number that is different from X. Each digit must be rotated - we cannot choose to leave it alone.

A number is valid if each digit remains a digit after rotation. 0, 1, and 8 rotate to themselves; 2 and 5 rotate to each other (on this case they are rotated in a different direction, in other words 2 or 5 gets mirrored); 6 and 9 rotate to each other, and the rest of the numbers do not rotate to any other number and become invalid.

Now given a positive number N, how many numbers X from 1 to N are good?

**Example:**
Input: 10
Output: 4
Explanation:
There are four good numbers in the range [1, 10] : 2, 5, 6, 9.
Note that 1 and 10 are not good numbers, since they remain unchanged after rotating.

Note:

• N will be in range [1, 10000].

# @lc code=start
using LeetCode

function rotated_digits(N::Int)
    if N <= 9
        return count(==(1), [0, 1, -1, -1, 1, 1, -1, 0, 1][1:N])
    end
    ans, dp = 0, vcat([0, 0, 1, -1, -1, 1, 1, -1, 0, 1], fill(0, N - 9))
    for i in 1:N-1
        dp[i + 1] = (-1 in [dp[i ÷ 10 + 1], dp[i % 10 + 1]]) ? -1 : (dp[i ÷ 10 + 1] | dp[i % 10 + 1])
        ans += (dp[i + 1] == 1)
    end
    ans
end
# @lc code=end

rotated_digits (generic function with 1 method)

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