This challenge wants you to find the largest product in a 20x20 grid that is 4 numbers in all directions (up, down, left, right, and all diagonals). This was pretty straight forward, I went with a brute force method.
Program.csnamespace ProjectEuler_11 { internal class Program { private static int[,] _data = { {08,02,22,97,38,15,00,40,00,75,04,05,07,78,52,12,50,77,91,08}, {49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,04,56,62,00}, {81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,03,49,13,36,65}, {52,70,95,23,04,60,11,42,69,24,68,56,01,32,56,71,37,02,36,91}, {22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80}, {24,47,32,60,99,03,45,02,44,75,33,53,78,36,84,20,35,17,12,50}, {32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70}, {67,26,20,68,02,62,12,20,95,63,94,39,63,08,40,91,66,49,94,21}, {24,55,58,05,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72}, {21,36,23,09,75,00,76,44,20,45,35,14,00,61,33,97,34,31,33,95}, {78,17,53,28,22,75,31,67,15,94,03,80,04,62,16,14,09,53,56,92}, {16,39,05,42,96,35,31,47,55,58,88,24,00,17,54,24,36,29,85,57}, {86,56,00,48,35,71,89,07,05,44,44,37,44,60,21,58,51,54,17,58}, {19,80,81,68,05,94,47,69,28,73,92,13,86,52,17,77,04,89,55,40}, {04,52,08,83,97,35,99,16,07,97,57,32,16,26,26,79,33,27,98,66}, {88,36,68,87,57,62,20,72,03,46,33,67,46,55,12,32,63,93,53,69}, {04,42,16,73,38,25,39,11,24,94,72,18,08,46,29,32,40,62,76,36}, {20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,04,36,16}, {20,73,35,29,78,31,90,01,74,31,49,71,48,86,81,16,23,57,05,54}, {01,70,54,71,83,51,54,69,16,92,33,48,61,43,52,01,89,19,67,48} }; private static int GetLargerHorizontalProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct) { // make sure we have enough colums to get the number of values we need if (x + multipleCount >= cols) { return largestProduct; } int product = data[x, y]; for (int i = x + 1; i < multipleCount + x; i++) { product *= data[i, y]; } return product > largestProduct ? product : largestProduct; } private static int GetLargerLeftDiagnolProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct) { if (x + multipleCount >= cols || y - multipleCount < 0) { return largestProduct; } int product = data[x, y]; for (int i = x + 1, j = y - 1; i < x + multipleCount; i++, j--) { product *= data[i, j]; } return product > largestProduct ? product : largestProduct; } private static int GetLargerProduct(int multipleCount, int[,] data, int rows, int cols, int largestProduct, int x, int y) { largestProduct = GetLargerHorizontalProduct(multipleCount, data, x, y, rows, cols, largestProduct); largestProduct = GetLargerVerticalProduct(multipleCount, data, x, y, rows, cols, largestProduct); largestProduct = GetLargerRightDiagnolProduct(multipleCount, data, x, y, rows, cols, largestProduct); largestProduct = GetLargerLeftDiagnolProduct(multipleCount, data, x, y, rows, cols, largestProduct); return largestProduct; } private static int GetLargerRightDiagnolProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct) { // make sure we have enough room to get the number of values we need if (x + multipleCount >= cols || y + multipleCount >= rows) { return largestProduct; } int product = data[x, y]; for (int i = x + 1, j = y + 1; i < x + multipleCount; i++, j++) { product *= data[i, j]; } return product > largestProduct ? product : largestProduct; } private static int GetLargerVerticalProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct) { // make sure we have enough rows to get the number of values we need if (y + multipleCount >= rows) { return largestProduct; } int product = data[x, y]; for (int i = y + 1; i < y + multipleCount; i++) { product *= data[x, i]; } return product > largestProduct ? product : largestProduct; } private static void Main(string[] args) { const int multipleCount = 4; int rows = _data.GetLength(0); int cols = _data.GetLength(1); int largestProduct = 0; for (int x = 0; x < rows; x++) { for (int y = 0; y < cols; y++) { largestProduct = GetLargerProduct(multipleCount, _data, rows, cols, largestProduct, x, y); } } System.Console.WriteLine("Largest multiple found: {0}", largestProduct); } } }