155. Min Stack.cpp

/*
Link: https://leetcode.com/problems/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.
*/

class MinStack {
public:
    /** initialize your data structure here. */
    stack<pair<int, int>> st;
    MinStack() {
    }
    
    void push(int x) {
        if (st.empty() || st.top().second > x){
            st.push( pair<int, int>(x, x) );
        } else {
            int t = st.top().second;
            st.push( pair<int, int>(x, t));
        }
    }
    
    void pop() {
        st.pop();
    }
    
    int top() {
        return st.top().first;
    }
    
    int getMin() {
        if(st.empty())
            return -1;
        return st.top().second;
    }
};

/**
 * 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();
 */