Compare Version Numbers

Compare two version numbers version1 and version2.
If version1 > version2 return 1, if version1 < version2 return -1, otherwise return 0.

You may assume that the version strings are non-empty and contain only digits and the . character.
The . character does not represent a decimal point and is used to separate number sequences.
For instance, 2.5 is not "two and a half" or "half way to version three", it is the fifth second-level revision of the second first-level revision.

Here is an example of version numbers ordering:

0.1 < 1.1 < 1.2 < 13.37

思路：

1.空值判断

2.字符串变为字符串数组，split("\\.")

3.关键点：循环比较，而且不清楚数组的长度，采用“或”逻辑，while(i<len1||i<len2),给点比较的默认值（此题为非负数），注意每次遍历数组都需要判断是否越界。

4.三种结果的条件判断

 public int compareVersion(String version1, String version2) {                  if(version1==null||version2==null) return 0;                  String[] s1=version1.split("\\.");         String[] s2=version2.split("\\.");                 //  int a1=Integer.parseInt(s1[0]);        //  int a2=Integer.parseInt(s2[0]);         int i=0;                 while(i<s1.length||i<s2.length){                        int a1=0,a2=0;            if(i<s1.length){                a1=Integer.parseInt(s1[i]);            }            if(i<s2.length){                a2=Integer.parseInt(s2[i]);            }                        if(a1>a2) return 1;            else if(a1<a2) return -1;            else i++;        }                return 0;              }