Read the How to prepare for an interview series at my blog: http://se7so.blogspot.com/2014/01/how-to-prepare-for-interview.html

prob12.cpp

#include<vector>

#define STACK_IS_EMPTY -1

class CustomStack {
private:
	vector<int> stck;
	vector<int> mn;
public:

	int top() {
		if(stck.size() != 0) return stck[stck.size()-1];
		throw STACK_IS_EMPTY;
	}
	
	void pop() {
		if(stck.size() == 0) return;
		
		if(mn[mn.size()-1] == stck[stck.size()-1])
			mn.erase(mn.begin()+mn.size()-1);
			
		stck.erase(stck.begin()+stck.size()-1);
	}
	
	void push(int n) {
		stck.push_back(n);
		
		if(mn.size() == 0 || n <= mn[mn.size()-1])
			mn.push_back(n);
	}
	
	int size() {
		return stck.size();
	}
	
	int min() {
		if(mn.size() == 0) throw STACK_IS_EMPTY;
		
		return mn[mn.size()-1];
	}
	
	bool empty() {
		return size() == 0;
	}
};