Add Boyer-Moore Majority Vote algorithm

Author: 0xsatoshi99

Algorithms.Tests/Other/BoyerMooreMajorityVoteTests.cs

@@ -0,0 +1,88 @@
+using Algorithms.Other;
+using NUnit.Framework;
+using FluentAssertions;
+
+namespace Algorithms.Tests.Other;
+
+public class BoyerMooreMajorityVoteTests
+{
+    [Test]
+    public void FindMajority_SimpleMajority_ReturnsCorrectElement()
+    {
+        var nums = new[] { 3, 3, 4, 2, 3, 3, 3 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(3);
+    }
+
+    [Test]
+    public void FindMajority_AllSameElements_ReturnsThatElement()
+    {
+        var nums = new[] { 5, 5, 5, 5 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(5);
+    }
+
+    [Test]
+    public void FindMajority_NoMajority_ReturnsNull()
+    {
+        var nums = new[] { 1, 2, 3, 4 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().BeNull();
+    }
+
+    [Test]
+    public void FindMajority_EmptyArray_ReturnsNull()
+    {
+        var nums = Array.Empty<int>();
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().BeNull();
+    }
+
+    [Test]
+    public void FindMajority_NullArray_ReturnsNull()
+    {
+        int[]? nums = null;
+        var result = BoyerMooreMajorityVote.FindMajority(nums!);
+        result.Should().BeNull();
+    }
+
+    [Test]
+    public void FindMajority_SingleElement_ReturnsThatElement()
+    {
+        var nums = new[] { 7 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(7);
+    }
+
+    [Test]
+    public void FindMajority_MajorityAtEnd_ReturnsCorrectElement()
+    {
+        var nums = new[] { 1, 2, 2, 2, 2 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(2);
+    }
+
+    [Test]
+    public void FindMajority_MajorityAtStart_ReturnsCorrectElement()
+    {
+        var nums = new[] { 8, 8, 8, 8, 1, 2 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(8);
+    }
+
+    [Test]
+    public void FindMajority_NegativeNumbers_ReturnsCorrectElement()
+    {
+        var nums = new[] { -1, -1, -1, 2, 2 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().Be(-1);
+    }
+
+    [Test]
+    public void FindMajority_ExactlyHalf_ReturnsNull()
+    {
+        var nums = new[] { 1, 1, 2, 2 };
+        var result = BoyerMooreMajorityVote.FindMajority(nums);
+        result.Should().BeNull();
+    }
+}

Algorithms/Other/BoyerMooreMajorityVote.cs

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+namespace Algorithms.Other;
+
+/// <summary>
+/// Boyer-Moore Majority Vote algorithm.
+/// Finds element appearing more than n/2 times in O(n) time, O(1) space.
+/// </summary>
+public static class BoyerMooreMajorityVote
+{
+    /// <summary>
+    /// Finds the majority element.
+    /// </summary>
+    /// <param name="nums">Input array.</param>
+    /// <returns>Majority element or null.</returns>
+    public static int? FindMajority(int[] nums)
+    {
+        if (nums?.Length == 0) return null;
+
+        var candidate = FindCandidate(nums!);
+        return IsMajority(nums!, candidate) ? candidate : null;
+    }
+
+    private static int FindCandidate(int[] nums)
+    {
+        int candidate = nums[0], count = 1;
+
+        for (int i = 1; i < nums.Length; i++)
+        {
+            if (count == 0) candidate = nums[i];
+            count += nums[i] == candidate ? 1 : -1;
+        }
+
+        return candidate;
+    }
+
+    private static bool IsMajority(int[] nums, int candidate) =>
+        nums.Count(n => n == candidate) > nums.Length / 2;
+}