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.

代碼：

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {}
    
    void push(int x) {
        s1.push(x);
        if (s2.empty() || x <= s2.top()) s2.push(x);
    }
    
    void pop() {
        if (s1.top() == s2.top()) s2.pop();
        s1.pop();
    }
    
    int top() {
        return s1.top();
    }
    
    int getMin() {
        return s2.top();
    }
    
private:
    stack<int> s1, s2;
};
/**
 * 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();
 */

　　用兩個棧來存 $s2$ 用來存最小的 第一次寫這樣的題 不是很會看了題解才知道怎麽寫 下次註意！

#Leetcode# 155. Min Stack