11問目

以下の20×20の数字の並びの中で, 対角線方向に4つの数字に赤く印が付けてあります.
(図省略)
それらの積は 26 × 63 × 78 × 14 = 1788696です.
20×20の数字の並びの中で, 任意の方向(上下左右もしくは斜め)の隣接する4つの数字の積の最大値はいくつか?

http://projecteuler.net/index.php?section=problems&id=11

8問目と同様力技以外に方法が無いと思います. 場合分けを適切に行うプログラミングの問題でした.

#include <stdio.h>

#define SIZE 20
#define NUM 4

enum direction {right, down, leftDown, rightDown};

int solveVer00();
int value(int, int, enum direction);
int max(int, int);

int numbers[SIZE][SIZE] = {{8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8},
{49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0},
{81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65},
{52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 1, 32, 56, 71, 37, 2, 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, 3, 45, 2, 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, 2, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21},
{24, 55, 58, 5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
{21, 36, 23, 9, 75, 0, 76, 44, 20, 45, 35, 14, 0, 61, 33, 97, 34, 31, 33, 95},
{78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 3, 80, 4, 62, 16, 14, 9, 53, 56, 92},
{16, 39, 5, 42, 96, 35, 31, 47, 55, 58, 88, 24, 0, 17, 54, 24, 36, 29, 85, 57},
{86, 56, 0, 48, 35, 71, 89, 7, 5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
{19, 80, 81, 68, 5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 4, 89, 55, 40},
{4, 52, 8, 83, 97, 35, 99, 16, 7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
{88, 36, 68, 87, 57, 62, 20, 72, 3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
{4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 8, 46, 29, 32, 40, 62, 76, 36},
{20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 4, 36, 16},
{20, 73, 35, 29, 78, 31, 90, 1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 5, 54},
{1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 1, 89, 19, 67, 48}};


int main() {
	fprintf(stdout, "Solution: %d\n", solveVer00());
	return 0;
}

int solveVer00() {
	int i, j, maxValue;

	maxValue = 0;
	// right
	for (i = 0; i < SIZE; i++) {
		for (j = 0; j < SIZE; j++) {
			maxValue = max(maxValue, value(i, j, right));
			maxValue = max(maxValue, value(i, j, down));
			maxValue = max(maxValue, value(i, j, leftDown));
			maxValue = max(maxValue, value(i, j, rightDown));
		}
	}
	return maxValue;
}

int max(int num1, int num2) {
	return (num1 > num2 ? num1 : num2);
}

int value(int row, int col, enum direction dir) {
	switch (dir) {
	case right:
		if (row < 0
			|| row >= SIZE
			|| col < 0
			|| col > SIZE - NUM) {
			return 0;
		} else {
			return numbers[row][col]
				* numbers[row][col + 1]
				* numbers[row][col + 2]
				* numbers[row][col + 3];
		}
		break;
	case down:
		if (row < 0
			|| row > SIZE - NUM
			|| col < 0
			|| col >= SIZE) {
			return 0;
		} else {
			return numbers[row][col]
				* numbers[row + 1][col]
				* numbers[row + 2][col]
				* numbers[row + 3][col];
		}
		break;
	case leftDown:
		if (row < 0
			|| row > SIZE - NUM
			|| col < NUM - 1
			|| col >= SIZE) {
			return 0;
		} else {
			return numbers[row][col]
				* numbers[row + 1][col - 1]
				* numbers[row + 2][col - 2]
				* numbers[row + 3][col - 3];
		}
		break;
	case rightDown:
		if (row < 0
			|| row > SIZE - NUM
			|| col < 0
			|| col > SIZE - NUM) {
			return 0;
		} else {
			return numbers[row][col]
				* numbers[row + 1][col + 1]
				* numbers[row + 2][col + 2]
				* numbers[row + 3][col + 3];
		}
		break;
	default:
		break;
	}
	return 0;
}