Project Euler : add solution for problem 18 (Max Path Sum I) + test.

Project-Euler/Problem018.js

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+/**
+ * @file Provides solution for Project Euler Problem 18 - Maximum path sum I
+ * @author Eric Lavault {@link https://github.com/lvlte}
+ * @license MIT
+ */
+
+/**
+ * Problem 18 - Maximum path sum I
+ *
+ * @see {@link https://projecteuler.net/problem=18}
+ *
+ * By starting at the top of the triangle below and moving to adjacent numbers
+ * on the row below, the maximum total from top to bottom is 23 :
+ *
+ *                            3
+ *                           7 4
+ *                          2 4 6
+ *                         8 5 9 3
+ *
+ * That is, 3 + 7 + 4 + 9 = 23.
+ *
+ * Find the maximum total from top to bottom of the triangle below :
+ *
+ *                            75
+ *                           95 64
+ *                         17 47 82
+ *                        18 35 87 10
+ *                      20 04 82 47 65
+ *                     19 01 23 75 03 34
+ *                   88 02 77 73 07 63 67
+ *                  99 65 04 28 06 16 70 92
+ *                41 41 26 56 83 40 80 70 33
+ *               41 48 72 33 47 32 37 16 94 29
+ *             53 71 44 65 25 43 91 52 97 51 14
+ *            70 11 33 28 77 73 17 78 39 68 17 57
+ *          91 71 52 38 17 14 91 43 58 50 27 29 48
+ *         63 66 04 68 89 53 67 30 73 16 69 87 40 31
+ *       04 62 98 27 23 09 70 98 73 93 38 53 60 04 23
+ *
+ * NOTE: As there are only 16384 routes, it is possible to solve this problem
+ * by trying every route. However, Problem 67, is the same challenge with a
+ * triangle containing one-hundred rows; it cannot be solved by brute force,
+ * and requires a clever method! ;o)
+ */
+
+const triangle = `
+75
+95 64
+17 47 82
+18 35 87 10
+20 04 82 47 65
+19 01 23 75 03 34
+88 02 77 73 07 63 67
+99 65 04 28 06 16 70 92
+41 41 26 56 83 40 80 70 33
+41 48 72 33 47 32 37 16 94 29
+53 71 44 65 25 43 91 52 97 51 14
+70 11 33 28 77 73 17 78 39 68 17 57
+91 71 52 38 17 14 91 43 58 50 27 29 48
+63 66 04 68 89 53 67 30 73 16 69 87 40 31
+04 62 98 27 23 09 70 98 73 93 38 53 60 04 23
+`
+
+export const maxPathSum = function (grid = triangle) {
+  /**
+   * If we reduce the problem to its simplest form, considering :
+   *
+   *         7      -> The max sum depends on the two adjacent numbers below 7,
+   *        2 4        not 7 itself.
+   *
+   *    obviously 4 > 2 therefore the max sum is 7 + 4 = 11
+   *
+   *                       6
+   * Likewise, with :     4 6     6 > 4 therefore the max sum is 6 + 6 = 12
+   *
+   * Now, let's say we are given :
+   *
+   *         3
+   *        7 6
+   *       2 4 6
+   *
+   *    and we decompose it into sub-problems such that each one fits the simple
+   *    case above, we got :
+   *
+   *         .           .           3
+   *        7 .         . 6         ? ?
+   *       2 4 .       . 4 6       . . .
+   *
+   *    Again, considering any number, the best path depends on the two adjacent
+   *    numbers below it, not the number itself. That's why we have to compute
+   *    the max sum from bottom to top, replacing each number with the sum of
+   *    that number plus the greatest of the two adjacent numbers computed from
+   *    the previous row.
+   *
+   *          .          .              3              15
+   *        11 .        . 12    ->    11 12    ->    x   x
+   *       x  x .      . x  x        x  x  x        x  x  x
+   *
+   * We are simplifying a complicated problem by breaking it down into simpler
+   * sub-problems in a recursive manner, this is called Dynamic Programming.
+   */
+
+  grid = grid.split(/\r\n|\n/).filter(l => l).map(r => r.split(' ').map(n => +n))
+
+  for (let i = grid.length - 2; i >= 0; i--) {
+    for (let j = 0; j < grid[i].length; j++) {
+      grid[i][j] += Math.max(grid[i + 1][j], grid[i + 1][j + 1])
+    }
+  }
+
+  return grid[0][0]
+}

Project-Euler/test/Problem018.test.js

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+import { maxPathSum } from '../Problem018'
+
+const example = `
+3
+7 4
+2 4 6
+8 5 9 3
+`
+
+describe('Check Problem 18 - Maximum path sum I', () => {
+  it('Check example', () => {
+    expect(maxPathSum(triangle)).toBe(23)
+  })
+
+  it('Check solution', () => {
+    expect(maxPathSum()).toBe(1074)
+  })
+})