2629. Function Composition

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題目敘述

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

解題思路

Solution

/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function(functions) {
	return function(x) {
        functions.reverse().foreach(fn => {
            x = fn(x)
        });
        return x;
    }
};

/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */

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

function compose(functions: F[]): F {
	return function(x): number {
        functions.reverse().forEach((fn: F) => {
            x = fn(x);
        });
        return x;
    }
};

/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */