字符流中出现的第一个字符

package algorithms;

import java.util.HashMap;

public class FirstAppearingOnce {
    HashMap<Character,Integer> hm = new HashMap<>();
    StringBuffer sb = new StringBuffer();
    //Insert one char from stringstream
    public void Insert(char ch)
    {
        if(hm.containsKey(ch)) {
            hm.put(ch, hm.get(ch)+1);
        }else {
            hm.put(ch, 1);
        }
        sb.append(ch);
    }
  //return the first appearence once char in current stringstream
    public char FirstAppearingOnce_1()
    {
        char[] str = sb.toString().toCharArray();
        for(int i=0;i<str.length;i++) {
            if(hm.get(str[i])==1) {
                return str[i];
            }
        }
        return '#';
    }
}

其实也可以自己实现一个简易的hashmap用来解决该道题

因为char（8位）有256种取值  所以可以创建一个长度为256的数组  数组的下标即为ascii码值  数组的值即为出现的次数，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;
    }