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Problem solution · Java

Maximum Frequency Stack

Maximum Frequency Stack: a Java solution using stack-based processing. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Stack-based processing
Source
walkccc LeetCode Solutions
Length
21 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Stack-based processing

For Maximum Frequency Stack, the implementation keeps unresolved items in last-in, first-out order, often to match boundaries, parse structure, or maintain monotonic candidates.

  1. Define what every stack entry represents.
  2. Pop entries once the current item resolves or invalidates them.
  3. Push the current item with only the information later steps need.

Code notes

  • 21 lines of Java from the credited upstream file 895.java.
  • The implementation visibly relies on hash lookup, ordered lookup, work queue.
  • No explicit loop blocks detected.

Complexity

If each item is pushed and popped at most once, the stack work is linear.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeMaximum Frequency Stack · JavaJava
Use this to learn the idea, then write your own version.
class FreqStack {  public void push(int val) {    count.merge(val, 1, Integer::sum);    countToStack.putIfAbsent(count.get(val), new ArrayDeque<>());    countToStack.get(count.get(val)).push(val);    maxFreq = Math.max(maxFreq, count.get(val));  }   public int pop() {    final int val = countToStack.get(maxFreq).pop();    count.merge(val, -1, Integer::sum);    if (countToStack.get(maxFreq).isEmpty())      --maxFreq;    return val;  }   private int maxFreq = 0;  private Map<Integer, Integer> count = new HashMap<>();  private Map<Integer, Deque<Integer>> countToStack = new HashMap<>();}

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