Package g0801_0900.s0895_maximum_frequency_stack

Class FreqStack

  • public class FreqStack
extends Object
    895 - Maximum Frequency Stack.

    Hard

    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.

    • Constructor Detail

      • FreqStack

        public FreqStack()

    • Method Detail

      • push

        public void push​(int val)

      • pop

        public int pop()