# # 【LeetCode】895. Maximum Frequency Stack 解题报告（Python）

## # 题目描述：

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:

1. Calls to FreqStack.push(int x) will be such that 0 <= x <= 10^9.
2. It is guaranteed that FreqStack.pop() won't be called if the stack has zero elements.
3. The total number of FreqStack.push calls will not exceed 10000 in a single test case.
4. The total number of FreqStack.pop calls will not exceed 10000 in a single test case.
5. The total number of FreqStack.push and FreqStack.pop calls will not exceed 150000 across all test cases.

## # 解题方法

``````我觉得是否有个问题？因为pop的过程中并没有更正已经进堆的那些数字的频率，也就是堆里面仍然是以前的频率。这里是否有问题？
``````

``````class FreqStack(object):

def __init__(self):
self.m = collections.defaultdict(int)
self.q = []
self.index = 0

def push(self, x):
"""
:type x: int
:rtype: void
"""
self.m[x] += 1
heapq.heappush(self.q, (-self.m[x], -self.index, x))
self.index += 1

def pop(self):
"""
:rtype: int
"""
val = heapq.heappop(self.q)[2]
self.m[val] -= 1
return val

# Your FreqStack object will be instantiated and called as such:
# obj = FreqStack()
# obj.push(x)
# param_2 = obj.pop()
``````

https://leetcode.com/problems/maximum-frequency-stack/discuss/163416/Java-Priority-Queue-easy-understand

## # 日期

2018 年 9 月 18 日 —— 铭记这一天