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.

class MinStack {
public:
stack<int> y1;
stack<int> y2;
void push(int x) {
y1.push(x);
if((y2.empty())||(x<y2.top()))
y2.push(x);
else
{
int z=y2.top();
y2.push(z);
}

}

void pop() {
y2.pop();
y1.pop();

}

int top() {
return y1.top();

}

int getMin() {
return y2.top();
}
};