155. Min Stack 最小栈

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.

设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。 push(x) -- 将元素 x 推入栈中。 pop() -- 删除栈顶的元素。 top() -- 获取栈顶元素。 getMin() -- 检索栈中的最小元素。 示例: MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); --> 返回 -3. minStack.pop(); minStack.top(); --> 返回 0. minStack.getMin(); --> 返回 -2.

思路：当minstk为空时，minstk.push(x)；当其不为空时，minstk.push(min(minstk.top(),x))；也就是说不空的时候输入当前元素和最小栈栈顶元素的最小值。

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

/**
 * 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();
 */