查找字符串中第一个非重复字符

题目描述：编写一个高效函数，找到字符串中首个非重复字符。如"total"首个非重复字符为'o'，"teeter"为'r'。（时间复杂度最好为o(n)）。代码如下：

package cn.lifx.test;

import java.util.Hashtable;

public class FindFirstUnique 
{
	private static final int N = 26;
	private static final String letters = "abcdefghigklmnopqrstuvwxyz";
	
	public static void main(String[] args)
	{
		String str = "totalo";
		
		findFirstUnique(str);
		
		findFirstUnique2(str);
		
		findFirstUnique3(str);
	}
	
	public static void findFirstUnique(String str)
	{
		char ch = 'a';
		int index = 0;
		int[] a = new int[N];
		
		for(int i=0; i<a.length; i++)
			a[i] = 0;
		
		for(int i=0; i<str.length(); i++)
		{
			ch = str.charAt(i);
			
			index = 25 - 'z' + ch;
			
			a[index]++;
		}
		
		int min = Integer.MAX_VALUE;
		for(int i=0; i<a.length; i++)
		{
			if(a[i] == 1)
			{
				ch = letters.charAt(i);
				index = str.indexOf(ch);
				
				if(index < min)
					min = index;
			}
		}
		
		if(min < Integer.MAX_VALUE)
		{
			System.out.println(str.charAt(min));
		}
		else
		{
			System.out.println("字符串中没有唯一不同的字符！");
		}
	}
	
	public static void findFirstUnique2(String str)
	{
		char ch;
		int index;
		int lastindex;
		
		for(int i=0; i<str.length(); i++)
		{
			ch = str.charAt(i);
			index = str.indexOf(ch);
			lastindex = str.lastIndexOf(ch);
			
			if(index == lastindex)
			{
				System.out.println(ch);
				break;
			}
		}
	}
	
	public static void findFirstUnique3(String str)
	{ 
        Character c; 
        Object seenOnce = new Object(); 
        Object seenTwice = new Object(); 
        
        Hashtable<Character, Object> charHash = new Hashtable<Character, Object>(); 

        for(int i=0; i<str.length(); i++)
        { 
        	c = new Character(str.charAt(i)); 
            Object object = charHash.get(c); 
            if(object == null)
            { 
            	charHash.put(c, seenOnce); 
            } 
            else if(object.equals(seenOnce))
            { 
            	charHash.put(c, seenTwice); 
            } 
        } 

        for(int i=0; i<str.length(); i++)
        { 
        	c = new Character(str.charAt(i)); 
            if(charHash.get(c)==seenOnce)
            { 
            	System.out.println(c);
            	break;
            } 
        } 
	} 
}

第一种方法不通用，或者说有问题：对于大写字母就会出错了，另外对于其他字符比如#$%等也会出错。