# 2623. Memoize

###### tags: `leetcode 30 days js challenge` `Medium`

[2623. Memoize](https://leetcode.com/problems/memoize/)

### 題目描述

Given a function `fn`, return a **memoized** version of that function.

A **memoized** function is a function that will never be called twice with the same inputs. Instead it will return a cached value.

You can assume there are **3** possible input functions: `sum`, `fib`, and `factorial`.

* `sum` accepts two integers `a` and `b` and returns `a + b`.
* `fib` accepts a single integer `n` and returns `1` if `n <= 1` or `fib(n - 1) + fib(n - 2)` otherwise.
* `factorial` accepts a single integer `n` and returns `1` if `n <= 1` or `factorial(n - 1) * n` otherwise.

### 範例

**Example 1:**

Input
"sum"
["call","call","getCallCount","call","getCallCount"]
[[2,2],[2,2],[],[1,2],[]]
Output
[4,4,1,3,2]

Explanation
const sum = (a, b) => a + b;
const memoizedSum = memoize(sum);
memoizedSum(2, 2); // Returns 4. sum() was called as (2, 2) was not seen before.
memoizedSum(2, 2); // Returns 4. However sum() was not called because the same inputs were seen before.
// Total call count: 1
memoizedSum(1, 2); // Returns 3. sum() was called as (1, 2) was not seen before.
// Total call count: 2

**Example 2:**

Input
"factorial"
["call","call","call","getCallCount","call","getCallCount"]
[[2],[3],[2],[],[3],[]]
Output
[2,6,2,2,6,2]

Explanation
const factorial = (n) => (n <= 1) ? 1 : (n * factorial(n - 1));
const memoFactorial = memoize(factorial);
memoFactorial(2); // Returns 2.
memoFactorial(3); // Returns 6.
memoFactorial(2); // Returns 2. However factorial was not called because 2 was seen before.
// Total call count: 2
memoFactorial(3); // Returns 6. However factorial was not called because 3 was seen before.
// Total call count: 2

**Example 3:**

Input
"fib"
["call","getCallCount"]
[[5],[]]
Output
[8,1]

Explanation
fib(5) = 8
// Total call count: 1

**Constraints**:

- 0 <= a, b <= 10⁵
- `1 <= n <= 10`
- `at most 105 function calls`
- `at most 105 attempts to access callCount`
- `input function is sum, fib, or factorial`

### 解答

#### TypeScript

type Fn = (...params: any) => any;

function memoize(fn: Fn): Fn {
  const map = new Map();

  return (...args) => {
    const paramsKey = JSON.stringify(args);
    if (map.has(paramsKey)) {
      return map.get(paramsKey);
    }

    map.set(paramsKey, fn(...args));
    return map.get(paramsKey);
  };
}

// one liner
const memoize = (fn: Fn, cache = {}): Fn => (...args) => (cache[args.join()] = cache[args.join()] ?? fn(...args));

> [name=Sheep][time=Sat, May 13, 2023]

### Reference

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