7. reverse integer

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.

Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

solution

Use some math including modulus and integer division to build up the answer in reverse.

class Solution:
    def reverse(self, x: int) -> int:
        isNegative = False
        if x < 0:
            isNegative = True
            x = abs(x)
 
        ans = 0
        while x > 0:
            last_digit = x % 10
            ans *= 10
            ans += last_digit
 
            if ans > 2 ** 31: return 0
 
            x = x // 10
        return -ans if isNegative else ans

The above solution doesn’t account for overflow, so we can instead add some conditions to anticipate overflow and return early, very similar to the logic seen in the solution for 8-string-to-integer.

def reverse(self, x: int) -> int:
	INT_MAX = 2**31-1
	INT_MIN = -(2**31)
	  
	is_neg = False
	if x < 0:
		is_neg = True
		x *= -1
	  
	ans = 0
	while x > 0:
		num_to_add = x % 10
	  
		if ans > INT_MAX // 10:
			return 0
		if ans == INT_MAX // 10:
			if is_neg:
				if num_to_add > 8:
					return 0
			else:
				if num_to_add > 7:
					return 0
	  
		ans *= 10
		ans += num_to_add
	  
		# pop off last digit from x
		x //= 10
 
	return -ans if is_neg else ans