Tags: "leetcode", "stack", access_time 2-min read

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Max Stack

Design a max stack that supports push, pop, top, peekMax and popMax.
1. push(x) – Push element x onto stack. 2. pop() – Remove the element on top of the stack and return it. 3. top() – Get the element on the top. 4. peekMax() – Retrieve the maximum element in the stack. 5. popMax() – Retrieve the maximum element in the stack, and remove it. If you find more than one maximum elements, only remove the top-most one.

Example 1:

MaxStack stack = new MaxStack();
stack.push(5); 
stack.push(1);
stack.push(5);
stack.top(); -> 5
stack.popMax(); -> 5
stack.top(); -> 1
stack.peekMax(); -> 5
stack.pop(); -> 1
stack.top(); -> 5

Note:

  1. -1e7 <= x <= 1e7
  2. Number of operations won’t exceed 10000.
  3. The last four operations won’t be called when stack is empty.

Solution:

class MaxStack {

    Stack<Integer> stack;
    Stack<Integer> maxStack;
    /** initialize your data structure here. */
    public MaxStack() {
        stack = new Stack<>();
        maxStack = new Stack<>();
    }
    
    public void push(int x) {
        int max = maxStack.isEmpty() ? x : maxStack.peek();
        // Assign a max to every newcomer
        maxStack.push(max > x ? max : x);
        stack.push(x);
    }
    
    public int pop() {
        System.out.println(maxStack.pop());
        return stack.pop();
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int peekMax() {
        return maxStack.peek();
    }
    
    public int popMax() {
        int val = peekMax();
        Stack<Integer> buffer = new Stack();
        while (top() != val) { buffer.push(pop()); }
        pop();
        while (!buffer.isEmpty()) { push(buffer.pop()); }
        return val;
    }
}

/**
 * Your MaxStack object will be instantiated and called as such:
 * MaxStack obj = new MaxStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.peekMax();
 * int param_5 = obj.popMax();
 */

Time Complexity: O(N) for the popMax operation, and O(1) for the other operations, where N is the number of operations performed.
Space Complexity: O(N), the maximum size of the stack.