14. A portable navigation device, comprising: a database, comprising a set of keywords and a keyword table, wherein the keyword table records priority scores of the keywords and linkage strengths between the keywords; a speech recognition device, receiving voice commands; a screen, displaying the keywords and receiving user input triggered by a stylus or user's finger; a keyword buffer, storing the keyword selected by the voice commands or the user input triggered by a stylus or user's finger; a microprocessor, performing an initialization operation to initialize the set of keywords, sorting priority of the set of keywords according to results of the initialization operation and displaying the set of keywords on the screen according to results of the sorting, selecting at least one of the keywords according to the voice commands or the user input triggered by a stylus or user's finger, assigning a weighted score to the selected keyword, refreshing the keyword table wherein the priory score of the selected keyword is refreshed according to the weighted score of the selected keyword, generating a weighted factor for the selected keyword, transmitting the weighted score and the weighted factor of the selected keyword to the keywords related to the selected keyword, refreshing the priority scores of the keywords related to the selected keyword according to the weighted score and the weighted factor of the selected keyword and re-sorting the priority of the keywords accordingly, displaying the keywords on the screen according to results of the re-sorting, pushing the selected keyword to the keyword buffer, strengthening linkage strengths between the keywords stored in the keyword buffer and refreshing the priority scores of all keywords accordingly, and performing a reset operation to remove the selected keyword out of the keyword buffer while the selected keyword has been stored in the keyword buffer over a predetermined time period.
14. A portable navigation device, comprising: a database, comprising a set of keywords and a keyword table, wherein the keyword table records priority scores of the keywords and linkage strengths between the keywords; a speech recognition device, receiving voice commands; a screen, displaying the keywords and receiving user input triggered by a stylus or user's finger; a keyword buffer, storing the keyword selected by the voice commands or the user input triggered by a stylus or user's finger; a microprocessor, performing an initialization operation to initialize the set of keywords, sorting priority of the set of keywords according to results of the initialization operation and displaying the set of keywords on the screen according to results of the sorting, selecting at least one of the keywords according to the voice commands or the user input triggered by a stylus or user's finger, assigning a weighted score to the selected keyword, refreshing the keyword table wherein the priory score of the selected keyword is refreshed according to the weighted score of the selected keyword, generating a weighted factor for the selected keyword, transmitting the weighted score and the weighted factor of the selected keyword to the keywords related to the selected keyword, refreshing the priority scores of the keywords related to the selected keyword according to the weighted score and the weighted factor of the selected keyword and re-sorting the priority of the keywords accordingly, displaying the keywords on the screen according to results of the re-sorting, pushing the selected keyword to the keyword buffer, strengthening linkage strengths between the keywords stored in the keyword buffer and refreshing the priority scores of all keywords accordingly, and performing a reset operation to remove the selected keyword out of the keyword buffer while the selected keyword has been stored in the keyword buffer over a predetermined time period. 21. The portable navigation device as claimed in claim 14 , wherein the microprocessor selects at least one of the set of keywords based on the voice commands received by the speech recognition device or user input via other interfaces.