155. 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. */
    MinStack() {
        
    }
    
    void push(int x) {
        s1.push(x);
        if (s2.empty() || x <= getMin()) s2.push(x);
    }
    
    void pop() {
        if (s1.top() == s2.top()) s2.pop();
        s1.pop();
    }
    
    int top() {
        return s1.top();
    }
    
    int getMin() {
        return s2.top();
    }
private:
    stack<int> s1, s2; // s1: stores all number, s2: stores minimum nums
};

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() : minVal(INT_MAX) {
        
    }
    
    void push(int x) {
        if (x <= minVal) {
            st.push(minVal);
            minVal = x;
        }
        st.push(x);
    }
    
    void pop() {
        int t = st.top(); st.pop();
        if (t == minVal) {
            minVal = st.top(); st.pop();
        }
    }
    
    int top() {
        return st.top();
    }
    
    int getMin() {
        return minVal;
    }
private:
    int minVal;
    stack<int> st;
};