Maximum Frequency Stack.cpp

/*
Maximum Frequency Stack
=======================

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.
*/

class FreqStack
{
public:
  unordered_map<int, int> freq;
  unordered_map<int, stack<int>> groups;
  int maxFreq;

  FreqStack()
  {
    maxFreq = 0;
  }

  void push(int x)
  {
    int f = ++freq[x];
    maxFreq = max(f, maxFreq);

    stack<int> st;
    if (groups.find(f) != groups.end())
    {
      st = groups[f];
    }
    st.push(x);
    groups[f] = st;
  }

  int pop()
  {
    int x = groups[maxFreq].top();
    groups[maxFreq].pop();
    freq[x]--;
    if (groups[maxFreq].size() == 0)
    {
      groups.erase(maxFreq);
      maxFreq--;
    }
    return x;
  }
};

/**
 * Your FreqStack object will be instantiated and called as such:
 * FreqStack* obj = new FreqStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 */