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 integerval
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 topush
andpop
. - It is guaranteed that there will be at least one element in the stack before calling
pop
.
Ideas:
1. 利用count 来记录每个num 出现的次数总和, 并且一直update
2. 利用self.maxFreq 来记录当前num出现最对的次数,可能有tie
3. 利用self.freqs 来记录每个次数的nums, 并且用stack来记录先后顺序
4. 每次pop 之后要检查self.freqs[self.maxFreq] 是否为空,然后来update self.maxFreq.
Code:
class FreqStack: def __init__(self): self.count = collections.Counter() self.freqs = collections.defaultdict(list) self.maxFreq = 0 def push(self, val: int) -> None: self.count[val] += 1 val_count = self.count[val] self.freqs[val_count].append(val) self.maxFreq = max(self.maxFreq, val_count) def pop(self) -> int: val = self.freqs[self.maxFreq].pop() self.count[val] -= 1 if not self.freqs[self.maxFreq]: self.maxFreq -= 1 return val