用JAVA语言实现从终端读入字符集大小n,以及n个字符和n个权值,建立哈夫曼树。

好的，下面是完整的Java代码实现：

import java.util.*;

class Node {
    char c;
    int weight;
    Node left;
    Node right;
    
    public Node(char c, int weight) {
        this.c = c;
        this.weight = weight;
        left = null;
        right = null;
    }
}

public class HuffmanTree {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        char[] chars = new char[n];
        int[] weights = new int[n];
        for (int i = 0; i < n; i++) {
            chars[i] = scanner.next().charAt(0);
            weights[i] = scanner.nextInt();
        }
        PriorityQueue<Node> queue = new PriorityQueue<>(Comparator.comparingInt(node -> node.weight));
        for (int i = 0; i < n; i++) {
            queue.offer(new Node(chars[i], weights[i]));
        }
        while (queue.size() > 1) {
            Node left = queue.poll();
            Node right = queue.poll();
            Node parent = new Node('\0', left.weight + right.weight);
            parent.left = left;
            parent.right = right;
            queue.offer(parent);
        }
        Node root = queue.poll();
        printHuffmanTree(root, "");
    }
    
    public static void printHuffmanTree(Node root, String code) {
        if (root != null) {
            if (root.left == null && root.right == null) {
                System.out.println(root.c + ": " + code);
            }
            printHuffmanTree(root.left, code + "0");
            printHuffmanTree(root.right, code + "1");
        }
    }
}

在运行程序时，首先输入字符集大小n，然后输入n个字符和n个权值，程序会按照权值构建哈夫曼树，并输出每个字符对应的哈夫曼编码。