extra/distract-the-trainers/solution.py

@@ -1,48 +1,78 @@
 def solution(banana_list):
-    fewest_possible_number = len(banana_list)
-    memo = {}
-    for pairs in generate_trainer_pairs(list(range(len(banana_list)))):
-        possible_number = len(banana_list)
-        for i, j in pairs:
-            if check_loop(banana_list[i], banana_list[j], memo):
-                possible_number -= 2
-        if possible_number < fewest_possible_number:
-            fewest_possible_number = possible_number
-        if fewest_possible_number < 2: # early termination
-            break
-    return fewest_possible_number
+    loop_memo = {}
+    edge_map = {i: [] for i in range(len(banana_list))}
+
+    for i in range(len(banana_list)):
+        for j in range(i+1, len(banana_list)):
+            if check_loop(banana_list[i], banana_list[j], loop_memo):
+                edge_map[i] += [j]
+                edge_map[j] += [i]
+
+    return len(banana_list) - len(find_maximum_matching(edge_map))
+
+def find_maximum_matching(edge_map):
+    exposed_vertex_list = [i for i in edge_map]
+    matching = {}
+    while len(exposed_vertex_list) >= 2:
+        start = exposed_vertex_list.pop()
+        path, end = find_new_path(edge_map, start, exposed_vertex_list, matching, {})
+        if end in exposed_vertex_list:
+            update_matching(matching, start, path, end)
+            exposed_vertex_list.remove(end)
+    return matching
 
 import copy
-def generate_trainer_pairs(trainer_id_list):
-    if len(trainer_id_list) < 2:
-        yield []
-    else:
-        first_id = trainer_id_list[0]
-        for second_id in trainer_id_list[1:]:
-            reduced_list = copy.deepcopy(trainer_id_list)
-            seduced_list.remove(first_id)
-            reduced_list.remove(second_id)
-            for pairs in generate_trainer_pairs(reduced_list):
-                yield [(first_id, second_id)] + pairs
+def find_new_path(edge_map, curr, end_list, matching, color):
+    for neighbor in edge_map[curr]:
+        if neighbor in end_list:
+            return [], neighbor
+    color[curr] = True
+    for neighbor in edge_map[curr]:
+        if neighbor in color:
+            continue
+        temp_color = copy.deepcopy(color)
+        temp_color[neighbor] = True
+        new_path, end = find_new_path(edge_map, matching[neighbor], end_list, matching, temp_color)
+        return [(curr, neighbor)] + new_path, end
+    return [], None
+
+def update_matching(matching, start, path, end):
+    new_left = start
+    for left, right in path:
+        matching[new_left] = left
+        matching[left] = new_left
+        new_left = right
+    matching[new_left] = end
+    matching[end] = new_left
+
+# How check_loop works
+#  1. (ac, bc) => (2ac, bc-ac) = (a, b) => (2a, b-a)
+#    * a < b
+#  1. (ac, bc+d) => (2ac, bc+d-ac) = (2ac, (b-a)c+d)
+#    * a < b and 0 < d < c
+#  1. (a, 2n-a) => (2a, 2n-2a) = (a, n-a)
+#    * a < n
+#  1. therefore, (x, y) will loop iff (x + y) % 2 == 1
+#    * x and y are relative prime number
+def check_loop(x, y, memo):
+    if (x, y) in memo:
+        return memo[(x, y)]
+    return bin((x + y)/gcd(x, y)).count('1') != 1
+
+def gcd(m, n):
+    while n != 0:
+        t = m % n
+        m, n = n, t
+    return m
 
-def check_loop(a, b, memo):
-    if (a, b) in memo:
-        return memo[(a, b)]
-    while a != b:
-        if (a + b) % 2 == 1: # sum is odd
-            memo[(a, b)] = True
-            return True
-        if a > b:
-            a, b = (a - b)/2, b
-        else:
-            a, b = a, (b - a)/2
-    memo[(a, b)] = False
-    return False
 
 tests = [
         ([1], 1),
         ([1, 1], 2),
         ([1, 7, 3, 21, 13, 19], 0),
+        #([1 for i in range(100)], 100),
+        #([2**(i+1) - 1 for i in range(10)], 2),
+        #([2**(i+1) - 1 for i in range(20)], 2),
         ]
 
 for i, o in tests: