(https://leetcode.com/problems/maximum-frequency-stack/)
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.
- stack
class FreqStack {
private:
unordered_map<int, int> freq; //key, freq
unordered_map<int, stack<int>> m; //freq, stack intem
int maxFreq=0; //max freq
public:
void push(int x) {
freq[x]++;
maxFreq = max(maxFreq, freq[x]);
m[freq[x]].push(x);
return;
}
int pop() {
int ret = m[maxFreq].top();
m[maxFreq].pop();
if(m[maxFreq].size()==0) maxFreq--;
freq[ret]--;
return ret;
}
};