155. 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.

本题我创建了一个栈，每次都压入了min，代码如下：

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

后来看了答案，发现其实不用占用那么多的内存，代码如下：

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

/**
 * 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();
 */