Problem 11

日課の一問。

(略)
上の 20 × 20 の数字のなか、赤くマークされた数字の積は 26 × 63 × 78 × 14 = 1788696 となる。
上下左右斜めのいずれかの方向で連続する4つの数字の積のうち最大のものを求めよ。

Problem 11 - PukiWiki

横方向・縦方向・右斜め下方向・左斜め下方向のそれぞれについて走査を行う。
Cだと、08とかの表記が8進数定数と扱われてエラーになるので10の位の0は省いた。

#include <stdio.h>

int numbers[20][20] = {
    {  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 }
};

#define SIDE_LENGTH 20

void problem011()
{
    int ans, tmp;
    int i,j;
    int *n;

    /* solve this problem here */
    ans = 0; tmp = 0;
    for (i=0; i<SIDE_LENGTH; i++) { /* 横向きの走査 */
        n = numbers[i];
        for (j=0; j<SIDE_LENGTH-3; j++) {
            tmp = *(n+j) * *(n+j+1) * *(n+j+2) * *(n+j+3);
            if (tmp > ans) {
                ans = tmp;
            }
        }
    }
    for (i=0; i<SIDE_LENGTH-3; i++) { /* 縦向きの走査 */
        n = numbers[i];
        for (j=0; j<SIDE_LENGTH; j++) {
            tmp = *(n+j) * *(n+j+SIDE_LENGTH*1) * *(n+j+SIDE_LENGTH*2) * *(n+j+SIDE_LENGTH*3);
            if (tmp > ans) {
                ans = tmp;
            }
        }
    }
    for (i=0; i<SIDE_LENGTH-3; i++) { /* 右下向きの斜めの走査 */
        n = numbers[i];
        for (j=0; j<SIDE_LENGTH-3; j++) {
            tmp = *(n+j) 
                * *(n+j+1 +SIDE_LENGTH*1)
                * *(n+j+2 +SIDE_LENGTH*2)
                * *(n+j+3 +SIDE_LENGTH*3);
            if (tmp > ans) {
                ans = tmp;
            }
        }
    }
    for (i=3; i<SIDE_LENGTH; i++) { /* 左下向きの斜めの走査 */
        n = numbers[i];
        for (j=0; j<SIDE_LENGTH-3; j++) {
            tmp = *(n+j) 
                * *(n+j-1 +SIDE_LENGTH*1)
                * *(n+j-2 +SIDE_LENGTH*2)
                * *(n+j-3 +SIDE_LENGTH*3);
            if (tmp > ans) {
                ans = tmp;
            }
        }
    }
        
    printf("%s: answer = %d\n", __FUNCTION__, ans);
}

int main(int argc, char* argv[]) 
{
    problem011();
    return 0;
}

実行結果

$ ./problem011
problem011: answer = 70600674