sort = function sort(arr1, arr2) {
          if (arr1[0] === arr2[0]) {
            if (arr1.length > 1 && arr2.length > 1) {
              return sort(tail(arr1), tail(arr2));
            } else {
              return 0;
            }
          } else {
            return arr1[0] > arr2[0] ? 1 : -1;
          }
        }
Compares any number of TreeNode objects and returns
the minimum and maximum (starting/ending) nodes.

@return {array} Array with two TreeNode objects.