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();
*/