# [WIP] 895. Maximum Frequency Stack

## Question

Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack.

Implement the `FreqStack`

class:

`FreqStack()`

constructs an empty frequency stack.`void push(int val)`

pushes an integer`val`

onto the top of the stack.`int pop()`

removes and returns the most frequent element in the stack.- If there is a tie for the most frequent element, the element closest to the stack's top 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

FreqStack freqStack = new FreqStack();

freqStack.push(5); // The stack is [5]

freqStack.push(7); // The stack is [5,7]

freqStack.push(5); // The stack is [5,7,5]

freqStack.push(7); // The stack is [5,7,5,7]

freqStack.push(4); // The stack is [5,7,5,7,4]

freqStack.push(5); // The stack is [5,7,5,7,4,5]

freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,5,7,4].

freqStack.pop(); // return 7, as 5 and 7 is the most frequent, but 7 is closest to the top. The stack becomes [5,7,5,4].

freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,4].

freqStack.pop(); // return 4, as 4, 5 and 7 is the most frequent, but 4 is closest to the top. The stack becomes [5,7].

Constraints:

`0 <= val <= 109`

At most 2 * 104 calls will be made to push and pop.

It is guaranteed that there will be at least one element in the stack before calling pop.

## Approach

## Solution

`class FreqStack {`

public:

unordered_map<int, int> freq;

unordered_map<int, stack<int>> m;

int maxfreq = 0;

FreqStack() {

}

void push(int val) {

maxfreq = max(maxfreq, ++freq[val]);

m[freq[val]].push(val);

}

int pop() {

int x = m[maxfreq].top();

m[maxfreq].pop();

if (!m[freq[x]--].size()) maxfreq--;

return x;

}

};

/**

* Your FreqStack object will be instantiated and called as such:

* FreqStack* obj = new FreqStack();

* obj->push(val);

* int param_2 = obj->pop();

*/