Problem

Given an integer array nums, a reducer function fn, and an initial value init, return the final result obtained by executing the fn function on each element of the array, sequentially, passing in the return value from the calculation on the preceding element.

This result is achieved through the following operations: val = fn(init, nums[0]), val = fn(val, nums[1]), val = fn(val, nums[2]), ... until every element in the array has been processed. The ultimate value of val is then returned.

If the length of the array is 0, the function should return init.

Please solve it without using the built-in Array.reduce method.

Examples

Example 1

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Input: 
nums = [1,2,3,4]
fn = function sum(accum, curr) { return accum + curr; }
init = 0
Output: 10
Explanation:
initially, the value is init=0.
(0) + nums[0] = 1
(1) + nums[1] = 3
(3) + nums[2] = 6
(6) + nums[3] = 10
The final answer is 10.

Example 2

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Input: 
nums = [1,2,3,4]
fn = function sum(accum, curr) { return accum + curr * curr; }
init = 100
Output: 130
Explanation:
initially, the value is init=100.
(100) + nums[0] * nums[0] = 101
(101) + nums[1] * nums[1] = 105
(105) + nums[2] * nums[2] = 114
(114) + nums[3] * nums[3] = 130
The final answer is 130.

Example 3

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Input: 
nums = []
fn = function sum(accum, curr) { return 0; }
init = 25
Output: 25
Explanation: For empty arrays, the answer is always init.

Constraints

  • 0 <= nums.length <= 1000
  • 0 <= nums[i] <= 1000
  • 0 <= init <= 1000

Solution

Method 1 – Custom Reduce Implementation

Intuition

Simulate the reduce operation by iteratively applying the reducer function to the accumulator and each element, starting from the initial value.

Approach

Define a function that takes nums, fn, and init. Start with ans = init, then for each element, update ans = fn(ans, num). Return ans at the end. If nums is empty, return init immediately.

Code

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function reduce(nums, fn, init) {
  let ans = init;
  for (let i = 0; i < nums.length; ++i) {
    ans = fn(ans, nums[i]);
  }
  return ans;
}

Complexity

  • ⏰ Time complexity: O(N)
  • 🧺 Space complexity: O(1)