包含min函数的栈

设计一个支持push，pop，top等操作并且可以在O(1)时间内检索出最小元素的堆栈。

push(x)–将元素x插入栈中
pop()–移除栈顶元素
top()–得到栈顶元素
getMin()–得到栈中最小元素
样例

MinStack minStack = new MinStack();
minStack.push(-1);
minStack.push(3);
minStack.push(-4);
minStack.getMin(); --> Returns -4.
minStack.pop();
minStack.top(); --> Returns 3.
minStack.getMin(); --> Returns -1.

class MinStack {
public:
    /** initialize your data structure here. */
    stack<int> normal_st;
    stack<int> min_st;
    
    MinStack() {
       
    }
    
    void push(int x) {
        if(normal_st.empty()){
            normal_st.push(x);
            min_st.push(x);
            return;
        }
        int tmp = min_st.top();
        if(x <= tmp){
            normal_st.push(x);
            min_st.push(x);
        }else{
            normal_st.push(x);
        }
    }
    
    void pop() {
        int tmp1 = min_st.top();
        int tmp2 = normal_st.top();
        if(tmp1 < tmp2){
            normal_st.pop();
        }
        else if(tmp1 == tmp2){
            normal_st.pop();
            min_st.pop();
        }
    }
    
    int top() {
        return normal_st.top();    
    }
    
    int getMin() {
        return min_st.top();
    }
};

/**
 * 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();
 */