1 package algorithms;
 2 
 3 import java.util.HashMap;
 4 
 5 public class FirstAppearingOnce {
 6     HashMap<Character,Integer> hm = new HashMap<>();
 7     StringBuffer sb = new StringBuffer();
 8     //Insert one char from stringstream
 9     public void Insert(char ch)
10     {
 
11         if(hm.containsKey(ch)) {
12             hm.put(ch, hm.get(ch)+1);
13         }else {
14             hm.put(ch, 1);
15         }
16         sb.append(ch);
17     }
18   //return the first appearence once char in current stringstream
19     public char FirstAppearingOnce_1()
20     {
 
21         char[] str = sb.toString().toCharArray();
22         for(int i=0;i<str.length;i++) {
23             if(hm.get(str[i])==1) {
24                 return str[i];
25             }
26         }
27         return ‘#‘;
28     }
29 }

其實也可以自己實現一個簡易的hashmap用來解決該道題

因為char（8位）有256種取值 所以可以創建一個長度為256的數組 數組的下標即為ascii碼值 數組的值即為出現的次數，如下所示：

int[] arr = new int[256];
    StringBuffer s = new StringBuffer();
    //Insert one char from stringstream
    public void Insert(char ch)
    {
        s.append(ch);
        arr[ch] +=1;
    }

字符流中出現的第一個字符