Isomorphic Strings
題意：
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 "egg", "add", return true.
Given "foo", "bar", return false.
Given "paper", "title", return true.
解題思路：
使用hashmap來幫助我們解決這道題，不斷的把s字元對應到相對應的t字元，一但發現不同時，即返回false，最後還需要檢查是否value的數目與key的數目相同。為了避免"ab"與"ab"的問題，其程式碼如下：
public class Solution {
public boolean isIsomorphic(String s, String t) {
HashMap<Character, Character> map = new HashMap<Character, Character>();
if (s == null || t == null || s.length() != t.length()) {
return false;
}
for (int i = 0; i < s.length(); i++) {
char cS = s.charAt(i);
char cT = t.charAt(i);
if (!map.containsKey(cS)) {
map.put(cS, cT);
} else {
if (map.get(cS) != cT) {
return false;
}
}
}
return map.values().size()== new HashSet<>(map.values()).size();
}
}