java - Java 中的速度为什么有些代码运行得更快

Question

我正在尝试 UVa 问题编号 10324我编写了以下解决方案，该解决方案被接受但给出了一个非常糟糕的 2.670 秒运行时间我有以下两个代码这是我的代码

    public static void main(String[] args) throws Exception{
        // write your code here
        StringBuilder op = new StringBuilder();
            InputStreamReader isr = new InputStreamReader(System.in);
            BufferedReader reader = new BufferedReader(isr);
            String input, tmp[];
            int n, l, u, j, i = 0;
            boolean dec;
            char thi;
            while ((input = reader.readLine()) != null) {
                op.append("Case " + (++i) + ":\n");
                n = Integer.parseInt(reader.readLine());
                while (n-- > 0) {
                    tmp = reader.readLine().split(" ");
                    l = Integer.parseInt(tmp[0]);
                    u = Integer.parseInt(tmp[1]);
                    if (l > u) {
                        l ^= u;
                        u ^= l;
                        l ^= u;
                    }
                    //System.out.println(l + "|" + u);
                    dec = true;
                    thi = input.charAt(l++);
                    for (; l <= u; l++) {
                        if (thi != input.charAt(l)||(thi != input.charAt(u--))) {
                            dec = false;
                            break;
                        }
                    }
                    op.append((dec ? "Yes\n" : "No\n"));
                }
            }
            System.out.print(op.toString());
            return;

    }
}

另一个代码是我在 Mr Gorgon's Solution 上找到的这有 0.84 秒的运行时间

public static void main(String[] args) throws Exception {
    InputStreamReader isr = new InputStreamReader(System.in);
    BufferedReader br = new BufferedReader(isr);
    StringBuilder sb = new StringBuilder("");
    int testCase = 1;
    String line;
    while ((line = br.readLine()) != null) {
        sb.append("Case ").append(testCase).append(":\n");
        testCase++;
        int noOfCases = Integer.parseInt(br.readLine());
        for (int j = 0; j < noOfCases; j++) {
            String[] str = br.readLine().split(" ");
            int val1 = Integer.parseInt(str[0]);
            int val2 = Integer.parseInt(str[1]);
            if (val1 > val2) {
                val1 ^= val2;
                val2 ^= val1;
                val1 ^= val2;
            }
            boolean isValid = true;
            if (val1 != val2) {
                for (int i = val1; i < val2; i++) {
                    if (line.charAt(i) != line.charAt(i + 1)) {
                        isValid = false;
                        break;
                    }
                }
            }
            if (isValid)
                sb.append("Yes\n");
            else
                sb.append("No\n");
        }
    }
    System.out.print(sb);
}

我发现很难理解为什么当所有任务基本相同时这段代码运行得如此快。而且我的代码比 Gorgons 代码有更少的声明

Answer 1

• op.append("案例"+ (++i) + ":\n");

这否定了 StringBuilder 的好处

• if (val1 != val2) { 在第二个版本中缩短了循环