Question

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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

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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Analysis

I’ve seen a couple of interesting ideas on other people’s blogs, including one that uses String.split() to convert input to an array, and this one which uses 2 pointers to compare.

My solution might seem more intuitive for some. See below for my solution.

Solution

The idea is to identify what the current number is (before end of string, or before the next ‘.’). If there’s no more string, value = 0, so the case of (1.0, 1) essentially equals. Without further ado, let’s look at the code.

Code

public class Solution {
    public int compareVersion(String version1, String version2) {
        if (version1 == null || version2 == null) {
            return 0;
        }
        return helper(version1, version2);
    }

    private int helper(String v1, String v2) {
        if (v1.length() == 0 && v2.length() == 0) {
            return 0;
        }

        int num1 = 0;
        int num2 = 0;

        if (v1.length() != 0) {
            int p1 = 0;
            while (p1 < v1.length() && v1.charAt(p1) != '.') {
                p1++;
            }
            num1 = Integer.parseInt(v1.substring(0, p1));
            if (p1 < v1.length()) p1++;
            v1 = v1.substring(p1);
        }

        if (v2.length() != 0) {
            int p2 = 0;
            while (p2 < v2.length() && v2.charAt(p2) != '.') {
                p2++;
            }
            num2 = Integer.parseInt(v2.substring(0, p2));
            if (p2 < v2.length()) p2++;
            v2 = v2.substring(p2);
        }

        if (num1 != num2) {
            return (num1 - num2) / Math.abs(num1 - num2);
        } else {
            return helper(v1, v2);
        }
    }
}