155. Min Stack

time: O(1)

space:O(n)

use two stack

class MinStack {
    
    // normal stack
    // min stack
    
    private Stack<Integer> stack;
    private Stack<Integer> min;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        min = new Stack<>();
    }
    
    public void push(int val) {
        stack.push(val);
        
        if (min.isEmpty()) {
            min.push(val);
        } else if (val <= min.peek()) {
            min.push(val);
        }
    }
    
    public void pop() {
        int x = stack.pop();
        if (x == min.peek()) {
            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(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */

use one stack

in the last step, the stack will be poped into an empty stack!

Integer.MAX_VALUE

class MinStack {

    private Stack<Integer> stack;
    private int min;
    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        min = Integer.MAX_VALUE;
    }
    
    public void push(int val) {
        if (val <= min) {
            stack.push(min);
            min = val;
        }
        stack.push(val);
    }
    
    public void pop() {
        if (stack.pop() == min) {
            min = stack.pop();
        }
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min;
    }
}

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