155. Min Stack

---

TIP

• Solving a question to implement stack using stack is possible , but not a good idea.
• you can use 2 vector, 1 vector, vector of pair or map to implement the stack.

Time Complexity: O(1) for all the solutions.

Space Complexity: Extra O(N) Space used.

---

Using 2 Vectors

• We maintain 2 vectors, 1 for stack implementation and another for min stack. push INT_MAX to min stack in declaration.
• push operation:
  • push element to stack.
  • push min element to min stack.
• pop operation:
  • pop element from stack.
  • pop min element from min stack.
• top operation:
  • return top element from stack.
• getMin operation:
  • return top element from min stack.

Code

class MinStack {
    vector<int> st;
    vector<int> mnStack;
public:
    MinStack() {
        mnStack.push_back(INT_MAX);
    }

    void push(int val) {
        st.push_back(val);
        mnStack.push_back(min(val,mnStack.back()));
    }

    void pop() {
        st.pop_back();
        mnStack.pop_back();
    }

    int top() {
        return st.back();
    }

    int getMin() {
        return mnStack.back();
    }
};

Using 1 vector

• Instead of 2 vectors, we can use 1 vector to implement the stack.
• push operation:
  • if minElement is greater or equal to val, push minElement to stack and update minElement to val.
  • then push val in the stack.
• pop operation:
  • pop the top of the stack.
  • if top is equal to minElement, minElement will be top(next top) of the stack and pop the top(next top) from stack.

Code

class MinStack{
    vector<int> st;
    int minElem;

public:
    MinStack() { minElem = INT_MAX; }

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

    void pop(){
        int top = st.back();
        st.pop_back();
        if (top == minElem){
            minElem = st.back();
            st.pop_back();
        }
    }

    int top() { return st.back(); }

    int getMin() { return minElem; }
};

Using vector of pair

• Self explanatory code

Code

class MinStack{
    vector<pair<int, int>> st;
public:
    MinStack() {}

    void push(int val){
        if (st.empty())
            st.push_back({val, val});
        else
            st.push_back({val, min(st.back().second, val)});
    }

    void pop() { st.pop_back(); }

    int top() { return st.back().first; }

    int getMin() { return st.back().second; }
};