现用一台2G内存4核CPU的电脑，尽可能快速的找出3.7亿个字中第一个不重复的字符，Java代码利用多线程分治实现

以下是Java代码，利用多线程分治实现查找第一个不重复的字符：

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.*;

public class FirstNonRepeatedCharacter {

    public static char findFirstNonRepeatedChar(String str) throws ExecutionException, InterruptedException {
        int numThreads = Runtime.getRuntime().availableProcessors();
        ExecutorService pool = Executors.newFixedThreadPool(numThreads);

        int chunkSize = str.length() / numThreads;
        int startIndex = 0;
        int endIndex = chunkSize;

        Map<Character, Integer> charCountMap = new HashMap<>();

        while (startIndex < str.length()) {
            if (endIndex > str.length()) {
                endIndex = str.length();
            }

            String chunk = str.substring(startIndex, endIndex);

            Future<Map<Character, Integer>> future = pool.submit(new CountCharsTask(chunk));
            Map<Character, Integer> chunkCountMap = future.get();

            for (Map.Entry<Character, Integer> entry : chunkCountMap.entrySet()) {
                char ch = entry.getKey();
                int count = entry.getValue();

                if (charCountMap.containsKey(ch)) {
                    charCountMap.put(ch, charCountMap.get(ch) + count);
                } else {
                    charCountMap.put(ch, count);
                }
            }

            startIndex += chunkSize;
            endIndex += chunkSize;
        }

        pool.shutdown();

        for (char ch : str.toCharArray()) {
            if (charCountMap.get(ch) == 1) {
                return ch;
            }
        }

        return Character.MIN_VALUE;
    }

    private static class CountCharsTask implements Callable<Map<Character, Integer>> {
        private final String chunk;

        public CountCharsTask(String chunk) {
            this.chunk = chunk;
        }

        @Override
        public Map<Character, Integer> call() throws Exception {
            Map<Character, Integer> countMap = new HashMap<>();

            for (char ch : chunk.toCharArray()) {
                if (countMap.containsKey(ch)) {
                    countMap.put(ch, countMap.get(ch) + 1);
                } else {
                    countMap.put(ch, 1);
                }
            }

            return countMap;
        }
    }

    public static void main(String[] args) {
        String str = "abacabadabacaba";
        try {
            char firstNonRepeatedChar = findFirstNonRepeatedChar(str);
            System.out.println("First non-repeated character: " + firstNonRepeatedChar);
        } catch (ExecutionException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

这个代码首先根据可用的处理器核心数创建一个固定大小的线程池。然后，将输入字符串分成多个块，每个块都由一个线程处理。每个线程计算块中每个字符出现的次数，并将其存储在一个映射中。最后，对所有映射进行合并，并查找第一个不重复的字符。如果找到了一个不重复的字符，则返回该字符；否则，返回`Character.MIN_VALUE`。