Confusing Number

Problem Description#

Source: https://leetcode.com/problems/confusing-number-ii/

We can rotate digits by 180 degrees to form new digits. When 0, 1, 6, 8, 9 are rotated 180 degrees, they become 0, 1, 9, 8, 6 respectively. When 2, 3, 4, 5 and 7 are rotated 180 degrees, they become invalid.

A confusing number is a number that when rotated 180 degrees becomes a different number with each digit valid.(Note that the rotated number can be greater than the original number.)

Given a positive integer N, return the number of confusing numbers between 1 and N inclusive.

Example 1:

Input: 20
Output: 6
Explanation: 
The confusing numbers are [6,9,10,16,18,19].
6 converts to 9.
9 converts to 6.
10 converts to 01 which is just 1.
16 converts to 91.
18 converts to 81.
19 converts to 61.

Example 2:

Input: 100
Output: 19
Explanation: 
The confusing numbers are [6,9,10,16,18,19,60,61,66,68,80,81,86,89,90,91,98,99,100].

Note: 1 <= N <= 10^9

Java
Python
C++

class Solution {
    Map<Integer, Integer> map;
    int count = 0;
    public Solution() {
        map = new HashMap<>();
        map.put(6, 9);
        map.put(9, 6);
        map.put(0, 0);
        map.put(1, 1);
        map.put(8, 8); 
    }

    private boolean isConfusingNumber(long n) {
        long src = n;
        long res = 0;
        while (n > 0) {
            res = res * 10 + map.get((int) n % 10); 
            n /= 10;
        }
        return res != src;
    }

    public int confusingNumberII(int N) {
        for(int x: new int[]{1, 6, 8, 9}){
            dfs(x, N);
        }
        return count;
    }

    public void dfs(long x, int N){
        if(x>N) return;
        if(isConfusingNumber(x)){
            count++;
        }
        for(int y: new int[]{0, 1, 6, 8, 9}){
            dfs(x*10 + y, N);
        }
    }
}