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.
【思路】两个栈实现，其中一个存入最小值。
class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
        
    }
    
    void push(int x) {
       stk.push(x); 
      if(minstk.empty()||x <=minstk.top())
       minstk.push(x);
    }
    
    void pop() {
        if(stk.top()==minstk.top())
        {
            stk.pop();
            minstk.pop();
        }else
         stk.pop();
    }
    
    int top() {
        return stk.top();
    }
    
    int getMin() {
       return minstk.top();
     
    }
private:
    stack<int> stk;
    stack<int> 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();
 */