Create friends-of-appropriate-ages.py

Author: kamyu104

Python/friends-of-appropriate-ages.py

@@ -0,0 +1,61 @@
+# Time:  O(a^2 + n), a is the number of ages,
+#                    n is the number of people
+# Space: O(a)
+
+# Some people will make friend requests.
+# The list of their ages is given and ages[i] is the age of the ith person.
+#
+# Person A will NOT friend request person B (B != A)
+# if any of the following conditions are true:
+#
+# age[B] <= 0.5 * age[A] + 7
+# age[B] > age[A]
+# age[B] > 100 && age[A] < 100
+# Otherwise, A will friend request B.
+#
+# Note that if A requests B, B does not necessarily request A.
+# Also, people will not friend request themselves.
+#
+# How many total friend requests are made?
+#
+# Example 1:
+#
+# Input: [16,16]
+# Output: 2
+# Explanation: 2 people friend request each other.
+# Example 2:
+#
+# Input: [16,17,18]
+# Output: 2
+# Explanation: Friend requests are made 17 -> 16, 18 -> 17.
+# Example 3:
+#
+# Input: [20,30,100,110,120]
+# Output:
+# Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.
+#
+# Notes:
+# - 1 <= ages.length <= 20000.
+# - 1 <= ages[i] <= 120.
+
+try:
+    xrange          # Python 2
+except NameError:
+    xrange = range  # Python 3
+
+import collections
+
+
+class Solution(object):
+    def numFriendRequests(self, ages):
+        """
+        :type ages: List[int]
+        :rtype: int
+        """
+        def request(a, b):
+            return 0.5*a+7 < b <= a
+
+        c = collections.Counter(ages)
+        return sum(int(request(a, b)) * c[a]*(c[b]-int(a == b))
+                   for a in c
+                   for b in c)