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

一刷
题解：
用string的自带函数split ‘.’， 然后同时比较小数点左边和右边

public class Solution {
    public int compareVersion(String version1, String version2) {
        String[] levels1 =  version1.split("\\.");
        String[] levels2 = version2.split("\\.");
        int length = Math.max(levels1.length, levels2.length);
        for(int i=0; i<length; i++){
            Integer v1 = i<levels1.length? Integer.parseInt(levels1[i]):0;
            Integer v2 = i<levels2.length? Integer.parseInt(levels2[i]):0;
            int compare = v1.compareTo(v2);
            if(compare!=0) return compare;
        }    
        return 0;
    }
}