15. A character string generation system including a processor and a memory for generating a character string to be displayed, the system comprising: a retrieving unit retrieving a document to be searched; a generating unit generating a frequency ordered suffix tree from the retrieved document, wherein the frequency ordered suffix tree is a list of each possible string of the document arranged according to a frequency of use of each possible string within the document; a searching unit searching the frequency ordered suffix tree on the basis of a keyword to retrieve a set context strings, wherein the set of context strings includes all of the each possible strings of the frequency ordered suffix tree beginning with the keyword, the retrieved set of context strings being a frequency ordered context tree as the context strings thereof are arranged according to frequency of use of the each possible string within the document; a compressing unit compressing the retrieved set of context strings of the frequency ordered context tree to optimize the frequency ordered context tree for display within a limited display region of a display device capable of displaying a limited number of context strings, the compressing including obtaining one or more context strings of the frequency ordered context tree to maximize a sum of areas under the condition that only a predetermined maximum number of context strings are displayed within the limited display region, wherein each area is defined as the frequency of use of a given context string multiplied by within the document multiplied by a length of the context string; and the display device displaying the compressed context strings of the frequency ordered context tree within the limited display region thereof.
15. A character string generation system including a processor and a memory for generating a character string to be displayed, the system comprising: a retrieving unit retrieving a document to be searched; a generating unit generating a frequency ordered suffix tree from the retrieved document, wherein the frequency ordered suffix tree is a list of each possible string of the document arranged according to a frequency of use of each possible string within the document; a searching unit searching the frequency ordered suffix tree on the basis of a keyword to retrieve a set context strings, wherein the set of context strings includes all of the each possible strings of the frequency ordered suffix tree beginning with the keyword, the retrieved set of context strings being a frequency ordered context tree as the context strings thereof are arranged according to frequency of use of the each possible string within the document; a compressing unit compressing the retrieved set of context strings of the frequency ordered context tree to optimize the frequency ordered context tree for display within a limited display region of a display device capable of displaying a limited number of context strings, the compressing including obtaining one or more context strings of the frequency ordered context tree to maximize a sum of areas under the condition that only a predetermined maximum number of context strings are displayed within the limited display region, wherein each area is defined as the frequency of use of a given context string multiplied by within the document multiplied by a length of the context string; and the display device displaying the compressed context strings of the frequency ordered context tree within the limited display region thereof. 20. The system according to claim 15 , wherein the number of the obtained one or more context strings does not exceed the length of the one or more predetermined maximum number of context strings, and wherein the number of the obtained one or more context strings does not exceed a number of characters in the limited display region.