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 integerx
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 that0 <= 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 exceed10000
in a single test case. - The total number of
FreqStack.pop
calls will not exceed10000
in a single test case. - The total number of
FreqStack.push
andFreqStack.pop
calls will not exceed150000
across all test cases.
Rust Solution
use std::collections::HashMap;
#[derive(Default)]
struct FreqStack {
freq: HashMap<i32, usize>,
stacks: HashMap<usize, Vec<i32>>,
max_freq: usize,
}
impl FreqStack {
fn new() -> Self {
FreqStack::default()
}
fn push(&mut self, x: i32) {
let n = self.freq.entry(x).or_default();
*n += 1;
self.stacks.entry(*n).or_default().push(x);
self.max_freq = self.max_freq.max(*n);
}
fn pop(&mut self) -> i32 {
let max_stack: &mut Vec<i32> = self.stacks.get_mut(&self.max_freq).unwrap();
let x = max_stack.pop().unwrap();
*self.freq.entry(x).or_default() -= 1;
if max_stack.is_empty() {
self.max_freq -= 1;
}
x
}
}
#[test]
fn test() {
let mut obj = FreqStack::new();
obj.push(5);
obj.push(7);
obj.push(5);
obj.push(7);
obj.push(4);
obj.push(5);
assert_eq!(obj.pop(), 5);
assert_eq!(obj.pop(), 7);
assert_eq!(obj.pop(), 5);
assert_eq!(obj.pop(), 4);
}
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