1056. Confusing Number

Problem Description

A confusing number is defined as an integer which, when rotated by 180 degrees, yields a different number, but still maintains its validity by only consisting of valid digits. Each digit has its rotated counterpart as follows:

0, 1, 8 remain unchanged when rotated (0 -> 0, 1 -> 1, 8 -> 8).
6 and 9 are swapped when rotated (6 -> 9, 9 -> 6).
Digits 2, 3, 4, 5, and 7 do not have valid rotations and thus make a number invalid if present after rotation.

When a number is rotated, we disregard leading zeros. For example, 8000 becomes 0008 after rotation, which we treat as 8.

The task is to determine whether a given integer n is a confusing number. If n is a confusing number, the function should return true; otherwise, it returns false.

Intuition

To solve this problem, we need to check two things:

Whether each digit in the given number has a valid rotation.
Whether the rotated number is different from the original number.

To start with, we map each digit to its rotated counterpart (if any), with invalid digits being mapped to -1. This gives us the array d with precomputed rotated values for all possible single digits:

[0, 1, -1, -1, -1, -1, 9, -1, 8, 6]

The intuition for the solution is to iterate through the digits of n from right to left, checking that each digit has a valid rotated counterpart, and simultaneously building the rotated number. This is achieved by the following steps:

Initialize two variables, x and y. x will hold the original number which we'll deconstruct digit by digit, and y will be used to construct the rotated number.
We use a loop to process each digit of x until all digits have been processed:
- x, v = divmod(x, 10) uses Python's divmod function to get the last digit v and update x to remove this last digit.
- We then check if the current digit v has a valid rotation by looking it up in the array d. If d[v] is -1, we have an invalid digit; in this case, we return false.
- If v is valid, we compute the new rotated digit and add it to y by shifting y to the left (by a factor of 10) and then adding d[v].
After processing all digits of x, we end up with y, which is the number formed after rotating n. We compare y with n to check if they are different. If they are the same, it means the number is not confusing and we return false. Otherwise, we return true.

Learn more about Math patterns.

Solution Approach

The implementation uses a simple algorithm that involves iterating through the digits of the given number to check for validity after rotation and building the rotated number at the same time.

Here's the breakdown:

Initialize variables: The solution starts with initializing two variables, x and y. x is assigned the value of the given number n and will be used to iterate through its digits. y is initialized to 0 and will be used to construct the rotated number.
Predefined rotations: A list d is created that defines the rotation of each digit. This list serves as a direct mapping, where the index represents the original digit and the value at that index represents the rotated digit. If a digit is invalid when rotated (e.g., 2, 3, 4, 5, or 7), its rotated value in the list is -1.
Iterate through digits: The while-loop is used to iterate through the digits of n from right to left. Inside the loop, divmod(x, 10) obtains the rightmost digit v of x and updates x to eliminate the rightmost digit. divmod is a Python built-in function that simultaneously performs integer division and modulo operation.
Validity check: The solution then checks for the validity of each digit by referencing the d list. If d[v] is -1, it means the digit v is invalid upon rotation, and the function returns false. An example is if the original number contains a 2, since 2 does not have a valid rotation equivalent.
Building rotated number: If the digit is valid, the rotated digit (found at d[v]) is added to y. To maintain the correct place value, y is first multiplied by 10 and then the rotated digit is added to it.
Final check: After processing the entire number, y would now be the rotated number. The rotated number y is then compared with the original number n. If they are identical, it means that the rotation has not changed the number, hence it is not a confusing number and the function returns false. Otherwise, it returns true.

This algorithm efficiently checks each digit of the number without the need for additional data structures and effectively builds the rotated number in-place using basic arithmetic operations.

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Example Walkthrough

Let's use the number 619 as a small example to illustrate the solution approach.

Initialize variables:
- x is assigned the value of n, so x becomes 619.
- y is initialized to 0.
Predefined rotations:
- We use the list d = [0, 1, -1, -1, -1, -1, 9, -1, 8, 6].
Iterate through digits:
- 619 has three digits, so we will perform the process below three times, once for each digit.
Validity check and building rotated number:
- First Iteration (rightmost digit 9):
  - x, v = divmod(619, 10) gives x = 61, v = 9.
  - d[v] = d[9] is 6 (valid rotation), so we move to building y.
  - y = y * 10 + d[v] = 0 * 10 + 6 = 6.
- Second Iteration (middle digit 1):
  - x, v = divmod(61, 10) gives x = 6, v = 1.
  - d[v] = d[1] is 1, 1 remains the same after rotation.
  - y = y * 10 + d[v] = 6 * 10 + 1 = 61.
- Third Iteration (leftmost digit 6):
  - x, v = divmod(6, 10) gives x = 0, v = 6 (as x is now less than 10).
  - d[v] = d[6] is 9 (valid rotation).
  - y = y * 10 + d[v] = 61 * 10 + 9 = 619.
Final check:
- Now x is 0, and we have finished processing the digits. We have y = 619.
- We compare y with the original n, and in this case, 619 is equal to 619, meaning the number did not change upon rotation.
- Since the rotated number is identical to the original number, 619 is not a confusing number according to our definition.

Therefore, the function would return false for 619 because rotating the number gives us the same number instead of a different number.

To see how a confusing number would work with this example, let's rotate the number 68:

Initialize x = 68 and y = 0.
x, v = divmod(68, 10) gives x = 6, v = 8.
- d[8] is 8, so y = 0 * 10 + 8 = 8.
x, v = divmod(6, 10) gives x = 0, v = 6.
- d[6] is 9, so y = 8 * 10 + 9 = 89.
The original number 68 is different from the rotated number 89, therefore the function would return true, indicating that 68 is indeed a confusing number.

Solution Implementation

1class Solution:
2    def confusingNumber(self, n: int) -> bool:
3        """
4        Determine if the given number is a confusing number. A confusing number is a number that,
5        when rotated 180 degrees, becomes a different valid number. If any digit cannot be rotated,
6        or the number remains the same after rotation, it is not a confusing number.
7
8        :param n: The input number to be tested.
9        :return: True if n is a confusing number, False otherwise.
10        """
11
12        # Original number and transformed/rotated number
13        original_number = n
14        rotated_number = 0
15
16        # Mapping of digits after 180-degree rotation
17        rotation_map = [0, 1, -1, -1, -1, -1, 9, -1, 8, 6]
18
19        # Process each digit of the original number
20        while original_number:
21            # Obtain the last digit and reduce the original number by one digit
22            original_number, last_digit = divmod(original_number, 10)
23
24            # Check for valid rotation, return False if rotation is invalid (indicated by -1)
25            if rotation_map[last_digit] < 0:
26                return False
27
28            # Build the rotated number by appending the rotated digit
29            rotated_number = rotated_number * 10 + rotation_map[last_digit]
30
31        # A number is confusing if it is different from its rotation
32        return rotated_number != n
33

1class Solution {
2    // Method to determine if a number is a confusing number
3    public boolean confusingNumber(int n) {
4        // Mappings from original digit to its possible flipped digit
5        // -1 indicates an invalid digit that doesn't have a valid transformation
6        int[] digitTransformations = new int[] {0, 1, -1, -1, -1, -1, 9, -1, 8, 6};
7      
8        // Original number
9        int originalNumber = n;
10        // Transformed number after flipping the digits
11        int transformedNumber = 0;
12      
13        // Process each digit of the original number
14        while (originalNumber > 0) {
15            // Get the last digit of the current number
16            int digit = originalNumber % 10;
17            // Check if the digit has a valid transformation
18            if (digitTransformations[digit] < 0) {
19                // If not, it's not a confusing number
20                return false;
21            }
22            // Update the transformed number with the flipped digit
23            transformedNumber = transformedNumber * 10 + digitTransformations[digit];
24            // Remove the last digit from the original number
25            originalNumber /= 10;
26        }
27        // The number is confusing if the transformed number is different from the original number
28        return transformedNumber != n;
29    }
30}
31

1class Solution {
2public:
3    // Function to check if a number is a confusing number
4    bool confusingNumber(int n) {
5        // Digit mapping, with -1 representing invalid mappings
6        vector<int> map = {0, 1, -1, -1, -1, -1, 9, -1, 8, 6};
7        int originalNumber = n; // Store the original number
8        int transformedNumber = 0; // Initialize the transformed number
9      
10        // Process each digit to create the transformed number
11        while (originalNumber) {
12            int digit = originalNumber % 10; // Get the last digit
13          
14            // Digit is not valid if it cannot be mapped
15            if (map[digit] < 0) {
16                return false; // This is not a confusing number
17            }
18          
19            // Build the transformed number by adding the mapped digit at the appropriate place
20            transformedNumber = transformedNumber * 10 + map[digit];
21          
22            // Remove the last digit from the original number for next iteration
23            originalNumber /= 10;
24        }
25      
26        // A number is confusing if it's not equal to the original number after transformation
27        return transformedNumber != n;
28    }
29};
30

1// Digit mapping, with undefined representing invalid mappings
2const digitMap: (number | undefined)[] = [0, 1, undefined, undefined, undefined, undefined, 9, undefined, 8, 6];
3
4// Function to check if a number is a confusing number
5function confusingNumber(n: number): boolean {
6    let originalNumber: number = n; // Store the original number
7    let transformedNumber: number = 0; // Initialize the transformed number
8
9    // Process each digit to create the transformed number
10    while (originalNumber > 0) {
11        const digit: number = originalNumber % 10; // Get the last digit
12      
13        // The digit is not valid if it cannot be mapped (i.e., it is undefined in digitMap)
14        if (digitMap[digit] === undefined) {
15            return false; // This is not a confusing number
16        }
17      
18        // Build the transformed number by adding the mapped digit at the appropriate place
19        transformedNumber = transformedNumber * 10 + digitMap[digit]!;
20      
21        // Remove the last digit from the original number for the next iteration
22        originalNumber = Math.floor(originalNumber / 10);
23    }
24  
25    // A number is confusing if it's not equal to the original number after transformation
26    return transformedNumber !== n;
27}
28

Time and Space Complexity

The time complexity of the given code is O(log n), where n is the input number. This complexity arises because the code processes each digit of the number exactly once, and there are O(log n) digits in a base-10 number.

The space complexity of the code is O(1) since it uses a constant amount of extra space regardless of the input size. The variables x, y, and v along with the array d are the only allocations, and their size does not scale with n.

Learn more about how to find time and space complexity quickly using problem constraints.

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