MaximumFrequencyStack895.java

/**
 * 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.
 */

public class MaximumFrequencyStack895 {
    /**
     * https://leetcode.com/problems/maximum-frequency-stack/solution/
     */
    class FreqStack {
        Map<Integer, Integer> freq;
        Map<Integer, Stack<Integer>> group;
        int maxfreq;

        public FreqStack() {
            freq = new HashMap();
            group = new HashMap();
            maxfreq = 0;
        }

        public void push(int x) {
            int f = freq.getOrDefault(x, 0) + 1;
            freq.put(x, f);
            if (f > maxfreq)
                maxfreq = f;

            group.computeIfAbsent(f, z-> new Stack()).push(x);
        }

        public int pop() {
            int x = group.get(maxfreq).pop();
            freq.put(x, freq.get(x) - 1);
            if (group.get(maxfreq).size() == 0)
                maxfreq--;
            return x;
        }
    }


/**
 * Your FreqStack object will be instantiated and called as such:
 * FreqStack obj = new FreqStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 */

}