LeetCode #895. Maximum Frequency Stack

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:

• 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 {
private:
    unordered_map<int,stack<int>> stacks;
    unordered_map<int,int> freq;
    int max_freq;
    
public:
    FreqStack() {
        max_freq=0;
    }
    
    void push(int x) {
        int f=freq[x];
        if(f==max_freq) max_freq++;
        f++;
        stacks[f].push(x);
        freq[x]=f;
    }
    
    int pop() {
        int x=stacks[max_freq].top();
        stacks[max_freq].pop();
        if(stacks[max_freq].size()==0)
        {
            stacks.erase(max_freq);
            max_freq--;
        }
        freq[x]--;
        return x;
    }
};