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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.

Solutions (Rust)

1. HashMap

use std::collections::HashMap;structFreqStack{num_freq:HashMap<i32,i32>,freq_stack:HashMap<i32,Vec<i32>>,max_freq:i32,}/**  * `&self` means the method takes an immutable reference. * If you need a mutable reference, change it to `&mut self` instead. */implFreqStack{fnnew() -> Self{Self{num_freq:HashMap::new(),freq_stack:HashMap::new(),max_freq:0,}}fnpush(&mutself,x:i32){let freq = self.num_freq.entry(x).or_default();*freq += 1;self.freq_stack.entry(*freq).or_default().push(x);self.max_freq = self.max_freq.max(*freq);}fnpop(&mutself) -> i32{let stack = self.freq_stack.get_mut(&self.max_freq).unwrap();let x = stack.pop().unwrap();*self.num_freq.get_mut(&x).unwrap() -= 1;if stack.is_empty(){self.max_freq -= 1;} x }}/** * Your FreqStack object will be instantiated and called as such: * let obj = FreqStack::new(); * obj.push(x); * let ret_2: i32 = obj.pop(); */
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