Reverse Integer python - python-3.x

Question

I am trying to solve the Leetcode "Reverse Integer" challenge in python.
I took look at the solution they supplied.
Their answer was written in Java.
I do not know why they use the following test-condition:
if (rev > Integer.MAX_VALUE/10 || (rev == Integer.MAX_VALUE / 10 && pop > 7)) return 0;
if (rev < Integer.MIN_VALUE/10 || (rev == Integer.MIN_VALUE / 10 && pop < -8)) return 0;
I wrote my python code without that same test condition.
It failed when I tested a negative number.
class Solution:
def reverse(self, x: int) -> int:
rev = 0
while( x != 0 ):
pop = x % 10
x //= 10
rev = rev * 10 +pop
print(rev)
return rev
I do not understand why that particular test-condition exists in their code.
The goal is to reverse the order of digits in an integer.
Some examples are shown below:
Example 1:
Input: x = 123
Output: 321
Example 2:
Input: x = -123
Output: -321
Example 3:
Input: x = 120
Output: 21
Example 4:
Input: x = 0
Output: 0
class Solution {
public int reverse(int x) {
int rev = 0;
while (x != 0) {
int pop = x % 10;
x /= 10;
if (rev > Integer.MAX_VALUE/10 || (rev == Integer.MAX_VALUE / 10 && pop > 7)) return 0;
if (rev < Integer.MIN_VALUE/10 || (rev == Integer.MIN_VALUE / 10 && pop < -8)) return 0;
rev = rev * 10 + pop;
}
return rev;
}
}

Answer 1

Branch conditions are sometimes easier to understand when they are drawn as tree-diagrams instead of one line formulas.
Also, we can write the tree like this:
├── OR
│ ├── (rev > Integer.MAX_VALUE/10)
│ └── AND
│ ├── rev == Integer.MAX_VALUE / 10
│ └── pop > 7
Imagine that rev is a container, like a cookie-jar, or a carboard box.
There is a maximum number of cookies which can be crammed into the cookie jar.
When rev == Integer.MAX_VALUE it means that the cookie jar is full.
The condition rev == Integer.MAX_VALUE/ 10 is represent the idea that the cookie-jar can only fit one more cookie.
Instead of stuffing cookies into cookie jars, we are stuffing digits (such as 5) into an integer.
The integer is the container.
Note that when you append zero to an integer in base 10, it is the same as multiplying by 10. For example, 25 becomes 250
Suppose that the largest integer allowed is 2,147,483,647
Note that 214,748,364 is equal to 2,147,483,647 / 10
When you divide by 10, the decimal part of the number is discarded.
When rev is 214,748,364, then you can append any integer the right-most end, as long as that digit is less than or equal to 7.
Old Element
New Element
New Container
Status
214748364
0
2147483640
VALID
214748364
1
2147483641
VALID
214748364
2
2147483642
VALID
214748364
3
2147483643
VALID
214748364
4
2147483644
VALID
214748364
5
2147483645
VALID
214748364
6
2147483646
VALID
214748364
7
2147483647
VALID
214748364
8
2147483648
TOO BIG
214748364
9
2147483649
TOO BIG
The "status" column in the table above
In their code, when they wrote (pop > 7) it probably should have been (pop > Integer.MAX_VALUE % 10)
Suppose that the largest integer allowed is 2,147,483,647
Then, 2,147,483,647 % 10 is equal to 7.
There are many, many different ways to reverse the digits of a number.
One solution, in python, is shown below:
x = int(reversed(str(x)))