[leetcode]7. Reverse Integer反转整数

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

题意：

给定一个10进制整数，翻转它。

Solution1: directly do the simulation. 

Two tricky parts to be handled:

(1) overflow :  32-bit signed integer range: [−2³¹,  2³¹ − 1],  whihc means [−2,147,483,648  2,147,483,647].  What if input is 2,147,483,647, after reversing, it will be 7,463,847,412.

(2) negative numbers: for each iteration, we do multiplication or division with a position number -- 10 , which means if sign is '-' , the sign will be kept all the time.

code:

/*
  Time Complexity:  O(log(n))  coz we just travese half part of original input
  Space Complexity: O(1) 
*/
class Solution {
    public int reverse(int input) {
        long sum = 0; 
        while(input !=0){
            sum = sum*10 + input %10;
            input = input /10;
            
            if(sum > Integer.MAX_VALUE || sum < Integer.MIN_VALUE){
                return 0;  // returns 0 when the reversed integer overflows    
            }
        }  
        return (int)sum;
    }
}