165. 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

比较两个版本号字符串。想法是从前往后读版本号，按点切分，一次读一个数，然后比较大小。对于版本号不一样长的情况,例如0.1和0.1.1，可给短的补0,相当于变成0.1.0和0.1.1，用一个over标志位表示是不是读完了，读完了，返回0 ，相当于补0。
为了格式整齐，在最开始给每个版本号最后加了一个'.',版本号就变成了1.1.1.这种形式。读到字符串结尾的\n的时候就可以把over标志位置位了。
代码如下：

class Solution {
public:
    int compareVersion(string version1, string version2) {
        int start1 = 0;
        int start2 = 0;
        version1 = version1 + ".";//为了格式整齐
        version2 = version2 + ".";
        bool over1 = false;
        bool over2 = false;
        while(over1 == 0 || over2 == 0)
        {
            int num1 = get_a_number(version1, start1, over1);
            int num2 = get_a_number(version2, start2, over2);
            if(num1 > num2)
                return 1;
            else if(num1 < num2)
                return -1;
        }
        return 0;
    }
    int get_a_number(string version, int & start, bool &over)
    {
        int num = 0;
        if(version[start] < '0' || version[start] > '9')
        {
            over = true;
            return 0;
        }
        while(version[start] >= '0' && version[start] <= '9')
        {
            num *= 10;
            num += (version[start] - '0');
            start++;
        }
        start ++;
        return num;
    } 
};