4. The document ranking system of claim 1 , wherein, in response to a presence of a link path having a plurality of steps of links from the second document to the first document, the contribution coefficient determining unit is configured to determine the contribution coefficient of the first document for contribution to the second document based on a contribution coefficient of a plurality of documents corresponding to the link path.
4. The document ranking system of claim 1 , wherein, in response to a presence of a link path having a plurality of steps of links from the second document to the first document, the contribution coefficient determining unit is configured to determine the contribution coefficient of the first document for contribution to the second document based on a contribution coefficient of a plurality of documents corresponding to the link path. 5. The document ranking system of claim 4 , wherein, in response to the first document being connected to the second document through a single link, the contribution coefficient determining unit is configured to determine the contribution coefficient of the first document for contribution to the second document based on the content score of the first document, a content score of the second document, and a content score of a third document connected by an outlink of the second document.