155. Min Stack (Easy)

https://leetcode.com/problems/min-stack/

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

  • push(x) -- Push element x onto stack.
  • pop() -- Removes the element on top of the stack.
  • top() -- Get the top element.
  • getMin() -- Retrieve the minimum element in the stack.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

Hints

Solutions

class MinStack {

    Stack<Integer> stack;
    Stack<Integer> min;
    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        min = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (min.isEmpty() || x <= min.peek()) {
            min.push(x);
        }
    }

    public void pop() {
        int value = stack.pop();
        if (min.peek() == value) {
            min.pop();
        }
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return min.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */
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