LeetCode 7. Reverse Integer

LeetCode problem No.7.

No.6 is here.

modalsoul.hatenablog.com

7. Reverse Integer

https://leetcode.com/problems/reverse-integer/

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

Note Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

Code 1

object Solution {
    def reverse(x: Int): Int = {
        var tmp = x
        var result = 0L
        while(tmp/10 != 0) {
          val digit = tmp%10
          result = result * 10 + digit
          tmp = tmp/10
        }
        result = result * 10 + tmp
        if(result < scala.Int.MinValue || result > scala.Int.MaxValue) 0 else result.toInt
    }
}

Methods

• Divide x by 10 and get remainder.
• Multiply current result by 10 and add remainder.
• When x becomes smaller than 10, exit while.
• If result is not in the the 32-bit signed integer range, result is 0.

Result

Runtime: 724 ms, faster than 5.95% of Scala online submissions for Reverse Integer. Memory Usage: 54.2 MB, less than 100.00% of Scala online submissions for Reverse Integer.

Very slowly...orz

Code 2

object Solution {
    def reverse(x: Int): Int = {
        var tmp = x
        var result = 0L
        Iterator.continually(tmp).takeWhile( n => n/10 != 0).foreach { n =>
          val digit = n%10
          result = result * 10 + digit
          tmp = tmp/10
        }
        result = result * 10 + tmp
        if(result < scala.Int.MinValue || result > scala.Int.MaxValue) 0 else result.toInt
    }
}

Methods

• Use Iterator.continually instead of while

Result

Runtime: 428 ms, faster than 5.95% of Scala online submissions for Reverse Integer. Memory Usage: 54.2 MB, less than 100.00% of Scala online submissions for Reverse Integer.

A little bit optimized, but not fast yet.

Code 3

object Solution {
    def reverse(x: Int): Int = {
        val result = if(x >= 0) x.toString.reverse.toLong else x.toString.reverse.init.toLong * -1
        if(result < scala.Int.MinValue || result > scala.Int.MaxValue) 0 else result.toInt
    }
}

Methods

• Convert Int to String
• Reverse String
• Convert String to Long
• If result is not in the the 32-bit signed integer range, result is 0.

Result

Runtime: 616 ms, faster than 5.95% of Scala online submissions for Reverse Integer. Memory Usage: 54.1 MB, less than 100.00% of Scala online submissions for Reverse Integer.

⛳!!