LeetCode 1088. Confusing Number II

原题链接在这里：https://leetcode.com/problems/confusing-number-ii/

题目：

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. 1 <= N <= 10^9

题解：

Create num and its rotation on the run with 0, 1, 6, 8, 9. If the number != rotation, count++.

num = num * 10 + d.

rotation = hm.get(d) *base + ratation.

base *= 10.

Time Complexity: exponential.

Space: O(N). stack space.

AC Java:

class Solution {
    int limit;
    int [] nums = new int[]{0, 1, 6, 8, 9};
    int count = 0;
    
    public int confusingNumberII(int N) {
        limit = N;
        
        HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
        hm.put(0, 0);
        hm.put(1, 1);
        hm.put(6, 9);
        hm.put(8, 8);
        hm.put(9, 6);
        
        dfs(0, 0, 1, hm); 
        return count;
    }
    
    private void dfs(long num, long reNum, long base, HashMap<Integer, Integer> hm){
        if(num > limit){
            return;
        }
        
        if(num != reNum){
            count++;
        }
        
        for(int d : nums){
            if(num == 0 && d == 0){
                continue;
            }
            
            dfs(num * 10 + d, hm.get(d) * base + reNum, base * 10, hm);
        }
    }
}

类似Confusing Number, Strobogrammatic Number III.