given two strings s and t, determine if they are isomorphic.
two strings are isomorphic if the characters in s can be replaced to get t.
all occurrences of a character must be replaced with another character while preserving the order of characters. no two characters may map to the same character but a character may map to itself.
for example,
given <code>"egg"</code>,<code>"add"</code>, return <code>true</code>.
given <code>"foo"</code>, <code>"bar"</code>, return <code>false</code>.
given <code>"paper"</code>, <code>"title"</code>, return <code>true</code>.
note:
you may assume both s and t have the same length.
建立兩個表：
①從字元串s到字元串t的字元映射表，因為最大的字母為122，是以映射表的大小為123。映射表鍵值為s中的字元，value為t中對應位置的字元。
②isused表代表t中的該字元是否已經被作為s中某字元的value值。