# 2625. Flatten Deeply Nested Array

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

[2625. Flatten Deeply Nested Array](https://leetcode.com/problems/flatten-deeply-nested-array/)

### 題目描述

Given a multi-dimensional array `arr` and a depth `n`, return a **flattened** version of that array.

A **multi-dimensional** array is a recursive data structure that contains integers or other **multi-dimensional** arrays.

A **flattened** array is a version of that array with some or all of the sub-arrays removed and replaced with the actual elements in that sub-array. This flattening operation should only be done if the current depth of nesting is less than `n`. The depth of the elements in the first array are considered to be `0`.

Please solve it without the built-in `Array.flat` method.

### 範例

**Example 1:**

Input
arr = [1, 2, 3, [4, 5, 6], [7, 8, [9, 10, 11], 12], [13, 14, 15]]
n = 0
Output
[1, 2, 3, [4, 5, 6], [7, 8, [9, 10, 11], 12], [13, 14, 15]]

Explanation
Passing a depth of n=0 will always result in the original array. This is because the smallest possible depth of a subarray (0) is not less than n=0. Thus, no subarray should be flattened. 

**Example 2:**

Input
arr = [1, 2, 3, [4, 5, 6], [7, 8, [9, 10, 11], 12], [13, 14, 15]]
n = 1
Output
[1, 2, 3, 4, 5, 6, 7, 8, [9, 10, 11], 12, 13, 14, 15]

Explanation
The subarrays starting with 4, 7, and 13 are all flattened. This is because their depth of 0 is less than 1. However [9, 10, 11] remains unflattened because its depth is 1.

**Example 3:**

Input
arr = [[1, 2, 3], [4, 5, 6], [7, 8, [9, 10, 11], 12], [13, 14, 15]]
n = 2
Output
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

Explanation
The maximum depth of any subarray is 1. Thus, all of them are flattened.

**Constraints**:

- 0 <= count of numbers in arr <= 10⁵
- 0 <= count of subarrays in arr <= 10⁵
- `maxDepth <= 1000`
- `-1000 <= each number <= 1000`
- `0 <= n <= 1000`

### 解答

#### TypeScript (recursive)

function flat(arr: MultiDimensionalArray, n: number): MultiDimensionalArray {
  const flattened: MultiDimensionalArray = [];

  for (const element of arr) {
    if (Array.isArray(element) && n > 0) {
      flat(element, n - 1).forEach((d) => flattened.push(d));
    } else {
      flattened.push(element);
    }
  }

  return flattened;
}

> [name=Sheep][time=Fri, May 26, 2023]

#### TypeScript (stack)

function flat(arr: MultiDimensionalArray, n: number): MultiDimensionalArray {
  const stack: [MultiDimensionalArray | number, number][] = arr.map((item) => [item, n]);

  const result: MultiDimensionalArray = [];

  while (stack.length > 0) {
    const [item, depth] = stack.pop() as [MultiDimensionalArray | number, number];
    if (Array.isArray(item) && depth > 0) {
      stack.push(...item.map((el) => [el, depth - 1] as [MultiDimensionalArray, number]));
    } else {
      result.push(item);
    }
  }

  return result.reverse();
}

> [name=Sheep][time=Fri, May 26, 2023]

### Reference

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