Day 89, ##, Min Stack

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.

--------------------------------------------------
with two stacks

class MinStack {
public:
    MinStack() {
        minElem = INT_MAX;
    }

    void push(int x) {
        st.push(x);
        if (x <= minElem) {
            minSt.push(x);
            minElem = x;
        }
    }

    void pop() {
        if (st.empty()) return;
        if (st.top() == minElem) {
            minSt.pop();
            if (!minSt.empty()) {
                minElem = minSt.top();
            }else {
                minElem = INT_MAX;
            }
        }
        st.pop();
    }

    int top() {
        if (st.empty()) return -1;
        return st.top();
    }

    int getMin() {
        return (int)minElem;
    }
    
private:
    stack<int> st;
    stack<int> minSt;
    long minElem;
};

with one stack

class MinStack {
public:
    MinStack() {
        minElem = INT_MAX;
    }

    void push(int x) {
        if (x <= minElem) {
            st.push(minElem);
            minElem = x;
        }
        st.push(x);
    }

    void pop() {
        if (st.empty()) return;
        
        if (st.top() == minElem) {
            st.pop();
            if (!st.empty()) {
                minElem = st.top();
                st.pop();
            }else {
                minElem = INT_MAX;
            }
        }else
            st.pop();
    }

    int top() {
        if (st.empty()) return -1;
        return st.top();
    }

    int getMin() {
        return (int)minElem;
    }
    
private:
    stack<int> st;
    long minElem;
};