将HashMap传递给递归方法不起作用？

Question

我声明HashMap用于记忆，并将其传递给递归方法，但看起来HashMap并没有真正存储数据，并且每次递归调用都将其重置。下面是一个示例：

    public static int fib(int n) {
        Map<Integer, Integer> memo = new HashMap<>();
        return fib(n, memo);
    }

    private static int fib(int n, Map<Integer, Integer> memo) {
        if (n < 0) {
            throw new IllegalArgumentException(
                "Index was negative. No such thing as a negative index in a series.");

        // base cases
        } else if (n == 0 || n == 1) {
            return n;
        }

        // see if we've already calculated this
        if (memo.containsKey(n)) {
            System.out.printf("grabbing memo[%d]\n", n);
            return memo.get(n);
        }

        System.out.printf("computing fib(%d)\n", n);
        int result = fib(n - 1) + fib(n - 2);

        // memoize
        memo.put(n, result);

        return result;
    }

输出如下

computing fib(10)
computing fib(9)
computing fib(8)
computing fib(7)
computing fib(6)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(6)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(7)
computing fib(6)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(8)
computing fib(7)
computing fib(6)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(6)
computing fib(5)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)
computing fib(3)
computing fib(2)
computing fib(4)
computing fib(3)
computing fib(2)
computing fib(2)

但是如果我将HashMap作为静态变量并在fib（）的外部声明它，那会很好，但这不是我想要的。谁能帮帮我？

谢谢。

Answer 1

fib（int，Map）方法实际上从未递归调用。您每次创建的地图都不会使用。

fib(n - 1, memo) + fib(n - 2, memo);