155.Min Stack.md

155.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.

解析

两个栈，长度维持一样，第一个栈放实际数值，第二个栈放对应位置的最大值。

代码

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