def validate(self):
        """
        Validates the heap. Returns True if the heap is a valid min-heap, and
        False otherwise.

        >>> h = MinHeap()
        >>> h._items = [1, 3, 5]
        >>> h.validate()
        True
        >>> h._items = [1, 3, 5, 8, 9, 6, 7, 11]
        >>> h.validate()
        True
        >>> h._items = [2, 3, 1, 7]
        >>> h.validate()
        False
        """
        if self._items[0] > self._items[1]:
            return False
        for i in range(1,len(self._items)/2):
            if self._items[i-1] > (self._items[2*i] or self._items[2*i+1]):
                return False
        return True