155. Min Stack.cpp

/*

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 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2
 

Constraints:

Methods pop, top and getMin operations will always be called on non-empty stacks.

*/


class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
    }
    
    void push(int x) {
        if ( min.empty() || ( x <= min.back() ) ) min.push_back( x );
        else min.push_back( min.back() );
        s.push_back( x );
    }
    
    void pop() {
        min.pop_back();
        s.pop_back();
    }
    
    int top() {
        return s.back();
    }
    
    int getMin() {
        return min.back();
    }
private:
    vector<int> s;
    vector<int> min;
};

/**
 * 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();
 */