895. Maximum Frequency Stack

895. 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.

解法： 

关键是想到用HashMap of Stacks的数据结构。

e.g. 

["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]

数据结构：
stackMap: <freq, 这个freq的stack> 注意同一个数如果多次加会在多个freq的slot里出现
3: [5]
2: [5] [7]
1:  [5] [7] [4]

freqMap: <数，freq>
5: 3
7: 2
4: 1

class FreqStack {
    HashMap<Integer, Integer> freqMap;
    HashMap<Integer, Stack<Integer>> stackMap;
    int maxFreq;
    public FreqStack() {
        freqMap = new HashMap<Integer, Integer>();
        stackMap = new HashMap<Integer, Stack<Integer>>();
        maxFreq = 0;
    }
    
    public void push(int x) {
        int newFreq = 1;
        if (freqMap.containsKey(x)) {
            newFreq = freqMap.get(x) + 1;
        }
        freqMap.put(x, newFreq);
        if (!stackMap.containsKey(newFreq)) {
            stackMap.put(newFreq, new Stack<Integer>()); 
        }
        stackMap.get(newFreq).push(x);
        maxFreq = Math.max(maxFreq, newFreq);
        
    }
    
    public int pop() {
        if (maxFreq == 0) {
            return -1;
        }
        
        Integer toRemove = stackMap.get(maxFreq).pop();
        int originalFreq = maxFreq; // maxFreq might be decreased, so keep an originalFreq to use
        
        if (stackMap.get(maxFreq).isEmpty()) {
            stackMap.remove(maxFreq);
            maxFreq--;
        }
        
        if (originalFreq != 1) {
            freqMap.put(toRemove, originalFreq - 1);
        } else {
            freqMap.remove(toRemove);
        }
        
        return toRemove;
    }
}