Implement a function to check if a binary tree is a binary search tree.

checkBST

function checkBST(node){

  lastVisited = null;

  function checkBSTRecurse (node){
      if (node === null) return true;
    
      if ( !checkBSTRecurse(node.left) ) return false;
    
      if (node.val < lastVisited) return false;
      lastVisited = node.val;
      
      if ( !checkBSTRecurse(node.right) ) return false;
    
      return true;
  }

  return checkBSTRecurse(node);
}

// var a = {val: 10, left:null, right: null};
// var b = {val: 9, left:null, right: null};
// var c = {val: 8, left:null, right: null};
// var d = {val: 1, left:null, right: null};
// var e = {val: 11, left:null, right: null};
// var f = {val: 12, left:null, right: null};
// var g = {val: 19, left:null, right: null};

// a.left = b;
// b.left = c;
// c.left = d;
// a.right = e;
// e.right = f;
// f.right = g;

// console.log(checkBST(a));