Create maximum-frequency-stack.py

Author: kamyu104

Python/maximum-frequency-stack.py

@@ -0,0 +1,81 @@
+# Time:  O(1)
+# Space: O(n)
+
+# Implement FreqStack,
+# a class which simulates the operation of a stack-like data structure.
+#
+# FreqStack has two functions:
+#
+# push(int x), which pushes an integer x onto the stack.
+# pop(), which removes and returns the most frequent element in the stack.
+# If there is a tie for most frequent element,
+# the element closest to the top of the stack is removed and returned.
+#
+# Example 1:
+#
+# Input: 
+# ["FreqStack","push","push","push","push","push","push","pop","pop","pop","pop"],
+# [[],[5],[7],[5],[7],[4],[5],[],[],[],[]]
+# Output: [null,null,null,null,null,null,null,5,7,5,4]
+# Explanation:
+# After making six .push operations, the stack is [5,7,5,7,4,5] from bottom to top.  Then:
+#
+# pop() -> returns 5, as 5 is the most frequent.
+# The stack becomes [5,7,5,7,4].
+#
+# pop() -> returns 7, as 5 and 7 is the most frequent, but 7 is closest to the top.
+# The stack becomes [5,7,5,4].
+#
+# pop() -> returns 5.
+# The stack becomes [5,7,4].
+#
+# pop() -> returns 4.
+# The stack becomes [5,7].
+#
+# Note:
+# - Calls to FreqStack.push(int x) will be such that 0 <= x <= 10^9.
+# - It is guaranteed that FreqStack.pop() won't be called if the stack has zero elements.
+# - The total number of FreqStack.push calls will not exceed 10000 in a single test case.
+# - The total number of FreqStack.pop calls will not exceed 10000 in a single test case.
+# - The total number of FreqStack.push and
+#   FreqStack.pop calls will not exceed 150000 across all test cases.
+
+import collections
+
+
+class FreqStack(object):
+
+    def __init__(self):
+        self.__freq = collections.Counter()
+        self.__group = collections.defaultdict(list)
+        self.__maxfreq = 0
+
+    def push(self, x):
+        """
+        :type x: int
+        :rtype: void
+        """
+        self.__freq[x] += 1
+        if self.__freq[x] > self.__maxfreq:
+            self.__maxfreq = self.__freq[x]
+        self.__group[self.__freq[x]].append(x)
+        
+
+    def pop(self):
+        """
+        :rtype: int
+        """
+        x = self.__group[self.__maxfreq].pop()
+        if not self.__group[self.__maxfreq]:
+            self.__group.pop(self.__maxfreq)
+            self.__maxfreq -= 1
+        self.__freq[x] -= 1
+        if not self.__freq[x]:
+            self.__freq.pop(x)
+        return x
+
+
+# Your FreqStack object will be instantiated and called as such:
+# obj = FreqStack()
+# obj.push(x)
+# param_2 = obj.pop()