Solve problem 150 by using prefix sum array. Nice.

python/e150.py

@@ -0,0 +1,61 @@
+def generate_triangle():
+    t = 0
+    ts = [[]]
+    i_in_row, max_in_row = 0, 1
+    for _ in range(1, 500_500 + 1):
+        t = (615_949 * t + 797_807) % 2**20
+        s = t - 2**19
+        ts[-1].append(s)
+        i_in_row += 1
+        if i_in_row == max_in_row:
+            i_in_row = 0
+            max_in_row += 1
+            ts.append([])
+    ts.pop() # remove empty list
+    return ts
+
+
+def array_to_prefix_sum_array(xs):
+    ys = []
+    current_sum = 0
+    for x in xs:
+        current_sum += x
+        ys.append(current_sum)
+    return ys
+
+
+def range_sum(prefix_sum, i, j):
+    if i == 0:
+        return prefix_sum[j]
+    else:
+        return prefix_sum[j] - prefix_sum[i - 1]
+
+
+def find_min_triangle(row_i, col_i, sum_triangle):
+    current_sum, min_sum = 0, float("inf")
+    left_i, right_i = col_i, col_i
+    while row_i < len(sum_triangle):
+        current_sum += range_sum(sum_triangle[row_i], left_i, right_i)
+        row_i += 1
+        right_i += 1
+        min_sum = min(current_sum, min_sum)
+    return min_sum
+
+
+def euler_150():
+    triangle = generate_triangle()
+    # triangle = [[15], [-14, -7], [20, -13, -5], [-3, 8, 23, -26], [1, -4 , -5, -18, 5], [-16, 31, 2, 9, 28, 3]]
+    sum_triangle = [array_to_prefix_sum_array(xs) for xs in triangle]
+
+    min_triangle = float("inf")
+    for row_i in range(len(triangle)):
+        for col_i in range(len(triangle[row_i])):
+            potential_min = find_min_triangle(row_i, col_i, sum_triangle)
+            min_triangle = min(min_triangle, potential_min)
+    return min_triangle
+
+
+if __name__ == "__main__":
+    solution = euler_150()
+    print("e150.py: " + str(solution))
+    assert solution == -271248680