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.

Solution:

class MinStack {
public:
    /** initialize your data structure here. */
    stack s;
    stack m;
    
    MinStack() {
        
    }
    
    void push(int x) {
        s.push(x);
        if(m.empty() || (m.top() >= x)){
            m.push(x);
        }
    }
    
    void pop() {
        if(!s.empty()){
            int t = s.top();
            s.pop();
            if(!m.empty() && m.top() >= t){
                m.pop();
            }
        }
    }
    
    int top() {
        return s.top();
    }
    
    int getMin() {
        return m.top();
    }
};

/**
 * 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();
 */