155. Min Stack

title

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.

solution

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

runtime: 56ms

一个stack负责push/pop/top操作，另一个栈负责存储栈当前最小值。