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NeetCode #949LC-2626EasyJavaScript
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#949 · #2626 · Array Reduce Transformation(数组归约运算)

📌 Problem Statement & Constraints

Given an integer array nums, a reducer fn, and an initial value init, return fn(...fn(fn(init, nums[0]), nums[1]), ...). The built-in Array.reduce must not be used.

💡 Core Algorithmic Approaches

  1. Keep an accumulator acc initialised to init.
  2. Walk the array left to right, updating acc = fn(acc, nums[i]) at each step.
  3. Return acc when the array is exhausted.
  4. Crucially, an empty array must still return init.

💻 Benchmark Python3 Implementation

var reduce = function(nums, fn, init) {
    let acc = init;
    for (let i = 0; i < nums.length; i++) {
        acc = fn(acc, nums[i]);               // fn(accumulator, current)
    }
    return acc;                               // empty array -> init
};

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(n): one pass over the array.
💾 Space Complexity
O(1): a single accumulator.

⚠️ Interview Pitfalls & Follow-ups

  • Swapping the callback arguments to fn(nums[i], acc): the reducer signature is (accumulator, current).
  • Returning 0 or undefined for an empty array: the required result is init.
  • Implementing it recursively without a base case: an empty array would recurse forever or return the wrong value.