Added C# solutions for Chapter 2 (Hash Maps and Sets)

csharp/Hash Maps and Sets/GeometricSequenceTriplets.cs

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+public class Solution
+{
+    public int GeometricSequenceTriplets(int[] nums, int r)
+    {
+        // The default value of 0 is returned when accessing a key that 
+        // doesn’t exist in the hash map. 
+        Dictionary<int, int> leftMap = new Dictionary<int, int>();
+        Dictionary<int, int> rightMap = new Dictionary<int, int>();
+        int count = 0;
+        // Populate 'right_map' with the frequency of each element in the array.
+        foreach (int x in nums) 
+        {
+            rightMap[x] = rightMap.GetValueOrDefault(x) + 1;
+        }
+        // Search for geometric triplets that have x as the center.
+        foreach (int x in nums)
+        {
+            // Decrement the frequency of x in 'right_map' since x is now being
+            // processed and is no longer to the right.
+            rightMap[x] = rightMap.GetValueOrDefault(x) - 1;
+            if (x % r == 0)
+            {
+                count += leftMap.GetValueOrDefault(x / r) * rightMap.GetValueOrDefault(x * r);
+            }
+            // Increment the frequency of x in 'left_map' since it'll be a part of the
+            // left side of the array once we iterate to the next value of x.
+            leftMap[x] = leftMap.GetValueOrDefault(x) + 1;
+        }
+
+        return count;
+    }
+}

csharp/Hash Maps and Sets/LongestChainOfConsecutiveNumbers.cs

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+public class Solution
+{
+    public int LongestChainOfConsecutiveNumbers(int[] nums)
+    {
+        if (nums == null || nums.Length == 0)
+        {
+            return 0;
+        }
+        HashSet<int> numSet = new HashSet<int>(nums);
+        int longestChain = 0;
+        foreach (int num in numSet)
+        {
+            // If the current number is the smallest number in its chain, search for
+            // the length of its chain.
+            if (!numSet.Contains(num - 1))
+            {
+                int currentNum = num;
+                int currentChain = 1;
+                // Continue to find the next consecutive numbers in the chain.
+                while (numSet.Contains(currentNum + 1))
+                {
+                    currentNum++;
+                    currentChain++;
+                }
+                longestChain = Math.Max(longestChain, currentChain);
+            }
+        }
+
+        return longestChain;
+    }
+}

csharp/Hash Maps and Sets/LongestChainOfConsecutiveNumbersBruteForce.cs

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+public class Solution
+{
+    public int LongestChainOfConsecutiveNumbersBruteForce(int[] nums)
+    {
+        if (nums == null || nums.Length == 0)
+        {
+            return 0;
+        }
+        int longestChain = 0;
+        // Look for chains of consecutive numbers that start from each number.
+        foreach (int num in nums)
+        {
+            int currentNum = num;
+            int currentChain = 1;
+            // Continue to find the next consecutive numbers in the chain.
+            while (nums.Contains(currentNum + 1))
+            {
+                currentNum++;
+                currentChain++;
+            }
+            longestChain = Math.Max(longestChain, currentChain);
+        }
+
+        return longestChain;
+    }
+}

csharp/Hash Maps and Sets/PairSumUnsorted.cs

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+public class Solution
+{
+    public int[] PairSumUnsorted(int[] nums, int target)
+    {
+        Dictionary<int, int> hashmap = new Dictionary<int, int>();
+        for (int i = 0; i < nums.Length; i++) 
+        {
+            if (hashmap.ContainsKey(target - nums[i]))
+            {
+                return [hashmap[target - nums[i]], i];
+            }
+
+            hashmap[nums[i]] = i;
+        }
+
+        return [];
+    }
+}

csharp/Hash Maps and Sets/PairSumUnsortedTwoPass.cs

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+public class Solution
+{
+    public int[] PairSumUnsortedTwoPass(int[] nums, int target)
+    {
+        Dictionary<int, int> numMap = new Dictionary<int, int>();
+        // First pass: Populate the hash map with each number and its 
+        // index
+        for (int i = 0; i < nums.Length; i++) 
+        {
+            numMap[nums[i]] = i;
+        }
+        // Second pass: Check for each number's complement in the hash map.
+        for (int i = 0; i < nums.Length; i++)
+        {
+            int complement = target - nums[i];
+            if (numMap.ContainsKey(complement) && numMap[complement] != i)
+            {
+                return [i, numMap[complement]];
+            }
+        }
+
+        return [];
+    }
+}

csharp/Hash Maps and Sets/VerifySudokuBoard.cs

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+public class Solution
+{
+    public bool VerifySudokuBoard(int[][] board)
+    {
+        // Create hash sets for each row, column, and subgrid to keep 
+        // track of numbers previously seen on any given row, column, or 
+        // subgrid.
+        HashSet<int>[] rowSets = new HashSet<int>[9];
+        HashSet<int>[] columnSets = new HashSet<int>[9];
+        HashSet<int>[,] subgridSets = new HashSet<int>[3, 3];
+
+        // Initializing row and column sets.
+        for (int i = 0; i < 9; i++) 
+        {
+            rowSets[i] = new HashSet<int>();
+            columnSets[i] = new HashSet<int>();
+        }
+
+        // Initializing the subgrid sets.
+        for (int i = 0; i < 3; i++) 
+        {
+            for (int j = 0; j < 3; j++)
+            {
+                subgridSets[i, j] = new HashSet<int>();
+            }
+        }
+
+        for (int r = 0; r < 9; r++) 
+        {
+            for (int c = 0; c < 9; c++)
+            {
+                int num = board[r][c];
+                if (num == 0)
+                {
+                    continue;
+                }
+                // Check if 'num' has been seen in the current row, 
+                // column, or subgrid.
+                if (rowSets[r].Contains(num))
+                {
+                    return false;
+                }
+                if (columnSets[c].Contains(num))
+                {
+                    return false;
+                }
+                if (subgridSets[r / 3, c / 3].Contains(num))
+                {
+                    return false;
+                }
+                // If we passed the above checks, mark this value as seen 
+                // by adding it to its corresponding hash sets.
+                rowSets[r].Add(num);
+                columnSets[c].Add(num);
+                subgridSets[r / 3, c / 3].Add(num);
+            }
+        }
+
+        return true;
+    }
+}

csharp/Hash Maps and Sets/ZeroStriping.cs

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+public class Solution
+{
+    public void ZeroStriping(int[][] matrix)
+    {
+        if (matrix == null || matrix.Length == 0 || matrix[0] == null || matrix[0].Length == 0) 
+        {
+            return;
+        }
+        int m = matrix.Length, n = matrix[0].Length;
+        // Check if the first row initially contains a zero.
+        bool firstRowHasZero = false;
+        for (int c = 0; c < n; c++)
+        {
+            if (matrix[0][c] == 0)
+            {
+                firstRowHasZero = true;
+                break;
+            }
+        }
+        // Check if the first column initially contains a zero.
+        bool firstColHasZero = false;
+        for (int r = 0; r < m; r++) 
+        {
+            if (matrix[r][0] == 0)
+            {
+                firstColHasZero = true;
+                break;
+            }
+        }
+        // Use the first row and column as markers. If an element in the 
+        // submatrix is zero, mark its corresponding row and column in the 
+        // first row and column as 0.
+        for (int r = 1; r < m; r++)
+        {
+            for (int c = 1; c < n; c++)
+            {
+                if (matrix[r][c] == 0)
+                {
+                    matrix[0][c] = 0;
+                    matrix[r][0] = 0;
+                }
+            }
+        }
+        // Update the submatrix using the markers in the first row and 
+        // column.
+        for (int r = 1; r < m; r++)
+        {
+            for (int c = 1; c < n; c++)
+            {
+                if (matrix[0][c] == 0 || matrix[r][0] == 0)
+                {
+                    matrix[r][c] = 0;
+                }
+            }
+        }
+        // If the first row had a zero initially, set all elements in the 
+        // first row to zero.
+        if (firstRowHasZero)
+        {
+            for (int c = 0; c < n; c++)
+            {
+                matrix[0][c] = 0;
+            }
+        }
+        // If the first column had a zero initially, set all elements in 
+        // the first column to zero.
+        if (firstColHasZero)
+        {
+            for (int r = 0; r < m; r++)
+            {
+                matrix[r][0] = 0;
+            }
+        }
+    }
+}

csharp/Hash Maps and Sets/ZeroStripingHashSets.cs

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+public class Solution
+{
+    public void ZeroStripingHashSets(int[][] matrix)
+    {
+        if (matrix == null || matrix.Length == 0 || matrix[0] == null || matrix[0].Length == 0) 
+        {
+            return;
+        }
+        int m = matrix.Length, n = matrix[0].Length;
+        HashSet<int> zeroRows = new HashSet<int>();
+        HashSet<int> zeroCols = new HashSet<int>();
+        // Pass 1: Traverse through the matrix to identify the rows and
+        // columns containing zeros and store their indexes in the 
+        // appropriate hash sets.
+        for (int r = 0; r < m; r++) 
+        {
+            for (int c = 0; c < n; c++)
+            {
+                if (matrix[r][c] == 0)
+                {
+                    zeroRows.Add(r);
+                    zeroCols.Add(c);
+                }
+            }
+        }
+        // Pass 2: Set any cell in the matrix to zero if its row index is 
+        // in 'zero_rows' or its column index is in 'zero_cols'.
+        for (int r = 0; r < m; r++)
+        {
+            for (int c = 0; c < n; c++)
+            {
+                if (zeroRows.Contains(r) || zeroCols.Contains(c))
+                {
+                    matrix[r][c] = 0;
+                }
+            }
+        }
+    }
+}