Reverse Integer

Medium

The problem

Reverse the digits of the integer x and return the result, keeping the sign. If the reversed number does not fit in a signed 32-bit integer (below -2,147,483,648 or above 2,147,483,647), return 0.

Example 1
Input: x = 123
Output: 321
Example 2
Input: x = -123
Output: -321
Example 3
Input: x = 120
Output: 21
The zero ends up at the front and disappears.
Example 4
Input: x = 1534236469
Output: 0
The reverse, 9646324351, is larger than 2,147,483,647.

Limits

-2,147,483,648 ≤ x ≤ 2,147,483,647

Write it in Go. Try for about 20 minutes on paper first, then open one hint at a time.

Try it here

Write Go. Common packages like fmt and sort are imported for you. Keep the function name and inputs the same.

Hints, one at a time

Nudge

Peel digits off with % 10 and push them on with ×10. Watch for overflow before it happens.

The idea

Loop digit = x % 10; x /= 10; check that rev·10 + digit stays within the 32-bit range before updating.

Target: O(log x) time

Go function shape

func reverse(x int) int

Reference solution

Tested with go test. Try it yourself first, then compare.

// Reverse: reverse the digits of a 32-bit integer, or 0 if the result would overflow.
// The overflow check happens BEFORE multiplying, because afterwards it is too late.
func Reverse(x int) int {
	rev := 0
	for x != 0 {
		digit := x % 10
		x /= 10
		if rev > math.MaxInt32/10 || (rev == math.MaxInt32/10 && digit > 7) {
			return 0
		}
		if rev < math.MinInt32/10 || (rev == math.MinInt32/10 && digit < -8) {
			return 0
		}
		rev = rev*10 + digit
	}
	return rev
}

Read the lesson: Bit Manipulation & Math