leetcode_gym/_problems/reverseInteger.md

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layout	title	last_solved	category	leetcode_url	status
page	Reverse Integer	2020-06-02	math	https://leetcode.com/problems/reverse-integer	Solved

Problem

Given a 32-bit signed integer, reverse digits of an integer.

Example 1: Input: 123 Output: 321

Example 2: Input: -123 Output: -321

Example 3: Input: 120 Output: 21

Solution

The tricky part of this question is handling the overflows. Check out this video:

{% highlight python %}

def reverse(self, num: int) -> int:

# handle negative case
neg = False
if num<0:
    neg = True
    num = abs(num)

rev = 0

while num>0:

    # pop last digit
    popped = num % 10
    num = math.floor(num / 10)

    # handle overflow
    # if (rev > 2**31 / 10) or (rev == 2**31 / 10 and popped>7): return 0
    # if (rev < -2**31 / 10) or (rev == -2**31 / 10 and popped<-8): return 0
    
    # this is good enough
    if (rev >= 2**31 / 10) : return 0
    if (rev <= -2**31 / 10) : return 0

    rev = rev * 10 + popped

    print(num, popped, rev)


return rev*-1 if neg else rev

{% endhighlight %}

2**31 = 2147483648 -2**31 = -2147483648

-2147483648 to 2147483647 Because of the range see thee last digit

This is what happens when it overflows

{% highlight text %} 153423646 9 9 15342364 6 96 1534236 4 964 153423 6 9646 15342 3 96463 1534 2 964632 153 4 9646324 15 3 96463243 1 5 964632435 0 1 9646324351 9646324351 {% endhighlight %}

This is what happens when we prevent the overflow {% highlight text %} 153423646 9 9 15342364 6 96 1534236 4 964 153423 6 9646 15342 3 96463 1534 2 964632 153 4 9646324 15 3 96463243 1 5 964632435 0 {% endhighlight %}