coding-practice/problems/0165-compare-version-numbers/csharp/Program.cs

// LeetCode 165: Compare Version Numbers
// 難度: Medium
// 日期: 2025-09-23

using System;

public class Solution
{
    // Two-pointer parser that compares segments without allocating intermediate arrays.
    public int CompareVersion(string version1, string version2)
    {
        var i = 0;
        var j = 0;

        while (i < version1.Length || j < version2.Length)
        {
            var segment1 = NextSegment(version1, ref i);
            var segment2 = NextSegment(version2, ref j);

            var comparison = CompareSegments(segment1, segment2);
            if (comparison != 0)
            {
                return comparison;
            }
        }

        return 0;
    }

    // Reads the next numeric segment (between dots) as a span and advances the current index.
    private static ReadOnlySpan<char> NextSegment(string version, ref int index)
    {
        if (index >= version.Length)
        {
            return ReadOnlySpan<char>.Empty;
        }

        var start = index;
        while (index < version.Length && version[index] != '.')
        {
            index++;
        }

        var segment = version.AsSpan(start, index - start);

        if (index < version.Length && version[index] == '.')
        {
            index++;
        }

        return segment;
    }

    // Compares two trimmed segments lexicographically to avoid integer overflow.
    private static int CompareSegments(ReadOnlySpan<char> left, ReadOnlySpan<char> right)
    {
        left = TrimLeadingZeros(left);
        right = TrimLeadingZeros(right);

        if (left.Length > right.Length)
        {
            return 1;
        }

        if (left.Length < right.Length)
        {
            return -1;
        }

        for (var i = 0; i < left.Length; i++)
        {
            if (left[i] > right[i])
            {
                return 1;
            }

            if (left[i] < right[i])
            {
                return -1;
            }
        }

        return 0;
    }

    private static ReadOnlySpan<char> TrimLeadingZeros(ReadOnlySpan<char> segment)
    {
        var index = 0;
        while (index < segment.Length && segment[index] == '0')
        {
            index++;
        }

        return index == segment.Length ? ReadOnlySpan<char>.Empty : segment[index..];
    }

    public int MyCompareVersion(string version1, string version2)
    {
        var v1 = version1.Split('.');
        var v2 = version2.Split('.');

        if (v1.Length >= v2.Length)
        {
            return CompareString(v1, v2);
        }

        return CompareString(v2, v1) * -1;
    }

    private static int CompareString(string[] longer, string[] shorter)
    {
        for (var i = 0; i < longer.Length; i++)
        {
            _ = int.TryParse(longer[i], out var num1);
            var num2 = i < shorter.Length && int.TryParse(shorter[i], out var parsed) ? parsed : 0;

            if (num1 > num2)
            {
                return 1;
            }

            if (num1 < num2)
            {
                return -1;
            }
        }

        return 0;
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        // optional: keep console app functional for manual verification
        if (args.Length == 2)
        {
            var solution = new Solution();
            var result = solution.CompareVersion(args[0], args[1]);
            Console.WriteLine(result);
        }
    }
}