最小栈

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.

code1:三个栈

class MinStack {
private:
    stack<int> normalStak;
    stack<int> minStack;
    stack<int> tmpStack;
public:
    /** initialize your data structure here. */
    MinStack() {}

    void push(int x)
    {
        normalStak.push(x);
        if(minStack.empty()||x<=minStack.top())
            minStack.push(x);
        else
        {
            while(!minStack.empty()&&minStack.top()<x)
            {
                tmpStack.push(minStack.top());
                minStack.pop();
            }
            minStack.push(x);
            while(!tmpStack.empty())
            {
                minStack.push(tmpStack.top());
                tmpStack.pop();
            }
        }
        return ;
    }

    void pop()
    {
        int val=normalStak.top();
        normalStak.pop();
        while(minStack.top()!=val)
        {
            tmpStack.push(minStack.top());
            minStack.pop();
        }
        minStack.pop();
        while(!tmpStack.empty())
        {
            minStack.push(tmpStack.top());
            tmpStack.pop();
        }
        return;
    }

    int top() {
        return normalStak.top();
    }

    int getMin() {
        return minStack.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();
 */

code2：两个栈，同步入栈

class MinStack {
private:
    stack<int> normalStak;
    stack<int> minStack;
public:
    /** initialize your data structure here. */
    MinStack() {}

    void push(int x)
    {
        normalStak.push(x);
        if(minStack.empty()||x<=minStack.top())
            minStack.push(x);
        else
            minStack.push(minStack.top());
        return ;
    }

    void pop()
    {
        if(!normalStak.empty())
        {
            normalStak.pop();
            minStack.pop();
        }
    }

    int top() {
        return normalStak.top();
    }

    int getMin() {
        return minStack.top();
    }
};

code3：两个栈，非同步入栈

class MinStack {
private:
    stack<int> normalStak;
    stack<int> minStack;
public:
    /** initialize your data structure here. */
    MinStack() {}

    void push(int x)
    {
        normalStak.push(x);
        if(minStack.empty()||x<=minStack.top())
            minStack.push(x);

        return ;
    }

    void pop()
    {
        if(!normalStak.empty())
        {
            int val=normalStak.top();
            normalStak.pop();
            if(val==minStack.top())
                minStack.pop();
        }
        return ;
    }

    int top() {
        return normalStak.top();
    }

    int getMin() {
        return minStack.top();
    }
};

 

来源：https://www.cnblogs.com/tianzeng/p/11930659.html