Function Composition 複合函數

Given an array of functions [f1, f2, f3, ..., fn], return a new function fn that is the function composition of the array of functions.

The function composition of [f(x), g(x), h(x)] is fn(x) = f(g(h(x))).

The function composition of an empty list of functions is the identity function f(x) = x.

You may assume each function in the array accepts one integer as input and returns one integer as output.

Example 1:

Input: functions = [x => x + 1, x => x * x, x => 2 * x], x = 4
Output: 65
Explanation:
Evaluating from right to left ...
Starting with x = 4.
2 * (4) = 8
(8) * (8) = 64
(64) + 1 = 65

Example 2:

Input: functions = [x => 10 * x, x => 10 * x, x => 10 * x], x = 1
Output: 1000
Explanation:
Evaluating from right to left ...
10 * (1) = 10
10 * (10) = 100
10 * (100) = 1000

Example 3:

Input: functions = [], x = 42
Output: 42
Explanation:
The composition of zero functions is the identity function

Constraints:

-1000 <= x <= 1000
0 <= functions.length <= 1000
all functions accept and return a single integer

解法:

這是在講給一個Function的陣列，每個元素都是一個帶數字並且回傳數字的function，會傳進每個計算公式的陣列

回傳一個帶要初始值的function，計算方式是從後面的fun計算之後拿這個數字在跟前面的每個做計算

type F = (num: number) => number;

const compose = (functions: F[]) => num => functions.reverse().reduce((pre, cur)=> cur(pre), num);