3. The method according to claim 1 , wherein extracting the hierarchical structure comprises: determining a first hierarchical structure according to the collected directory structure information; determining a second hierarchical structure according to the collected link information of the web documents; and combining the first hierarchical structure and the second hierarchical structure to determine the extracted hierarchical structure.
3. The method according to claim 1 , wherein extracting the hierarchical structure comprises: determining a first hierarchical structure according to the collected directory structure information; determining a second hierarchical structure according to the collected link information of the web documents; and combining the first hierarchical structure and the second hierarchical structure to determine the extracted hierarchical structure. 4. The method according to claim 3 , wherein: a first ancestor-descendant relationship in the first hierarchical structure is determined by analyzing the collected directory structure information, wherein a first web document among the web documents is determined as an ancestor of a second web document among the web documents, if the first web document is an index document and has a same directory path as the second web document; and a second ancestor-descendant relationship in the second hierarchical structure is determined by comparing the collected link information between the web documents, wherein a third web document among the web documents is determined as an ancestor of a fourth web document among the web documents, if an in-bound links set of the fourth web document is a subset of an out-bound links set of the third web document.