leetcode-155-min-stack

Description

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 {
    class MinNode{
        int min, val;
        public MinNode(int min, int val){
            this.min = min;
            this.val = val; 
        }
    }
    
    Stack<MinNode> st;
    int min; 
    public MinStack() {
        st = new Stack<MinNode>();
        min = Integer.MAX_VALUE; 
    }
    
    public void push(int x) {
        if(x <= min) min = x; 
        MinNode t = new MinNode(min, x); 
        st.push(t); 
    }
    
    public void pop() {
        if(!st.isEmpty()){
            MinNode t = st.pop(); 
			
			//need to reset the min to what it was before we saw this value
            if(!st.isEmpty()) min = st.peek().min; 
            else min = Integer.MAX_VALUE; 
        }
    }
    
    public int top() {
        if(st.isEmpty()) return -1; 
        return st.peek().val; 
    }
    
    public int getMin() {
        if(st.isEmpty()) return -1; 
        return st.peek().min; 
    }
}