题解 | 24点运算

import java.io.*;
import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    private static final Map<String, Integer> str2Int = new HashMap<>();
    private static final String[] int2Str = new String[] {"", "A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "k"};
    private static char[] offerOps = new char[] {'+', '-', '*', '/'};
    private static final List<int[]> orders = new ArrayList<>();
    private static final int[] order = new int[4];
    private static final boolean[] vis = new boolean[4];
    private static final int[] nums = new int[4];
    private static final char[] ops = new char[3];
    private static final StringBuilder sb = new StringBuilder();
    private static boolean end = false;
    static {
        str2Int.put("A", 1);
        str2Int.put("2", 2);
        str2Int.put("3", 3);
        str2Int.put("4", 4);
        str2Int.put("5", 5);
        str2Int.put("6", 6);
        str2Int.put("7", 7);
        str2Int.put("8", 8);
        str2Int.put("9", 9);
        str2Int.put("10", 10);
        str2Int.put("J", 11);
        str2Int.put("Q", 12);
        str2Int.put("K", 13);
    }

    public static void main(String[] args) throws Exception {
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        String[] cards = bf.readLine().split(" ");
        for (String card : cards)
            if ("joker".equals(card) || "JOKER".equals(card)) {
                System.out.print("ERROR");
                return;
            }
        for (int i = 0; i < 4; i++) nums[i] = str2Int.get(cards[i]);
        Arrays.sort(nums);
        dfs1(0);
        for (int i = 0; i < orders.size() && !end; i++) dfs2(orders.get(i), 0);
        if (sb.length() == 0) System.out.print("NONE");
        else System.out.print(sb);
    }

    private static void dfs1(int idx) {
        if (idx == 4) {
            orders.add(order.clone());
            return;
        }
        for (int i = 0; i < 4; i++)
            if (!vis[i] && (i == 0 || nums[i] != nums[i - 1] || vis[i - 1])) {
                vis[i] = true;
                order[idx] = nums[i];
                dfs1(idx + 1);
                vis[i] = false;
            }
    }

    private static void dfs2(int[] arr, int idx) {
        if (idx == 3) {
            int sum = arr[0];
            for (int i = 0; i < 3; i++) sum = compute(sum, arr[i + 1], ops[i]);
            if (sum == 24) {
                for (int i = 0; i < 3; i++) sb.append(int2Str[arr[i]]).append(ops[i]);
                sb.append(int2Str[arr[3]]);
                end = true;
            }
            return;
        }
        for (int i = 0; i < 4 && !end; i++) {
            ops[idx] = offerOps[i];
            dfs2(arr, idx + 1);
        }
    }

    private static int compute(int a, int b, char op) {
        if (op == '+') return a + b;
        else if (op == '-') return a - b;
        else if (op == '*') return a * b;
        else return a / b;
    }
}

}

这个测试系统是不是有问题，逗我玩呢，不如leetcode