Problem
Combine the rows of a 2D integer array into one array. Read rows from top to bottom and integers within each row from left to right. Skip empty rows. Preserve repeated integers and their order. Rows may have different lengths. There are exactly two array levels: rows contain only integers, with no null values or deeper lists.
Implement flatten_nested_list(values).
Input
values: The integer rows to combine, in order.
Output
Return all integers in row order. Return an empty array if there are no integers.
Constraints
0 ≤ number of rows ≤ 10,000. At most 10,000 integers in total. Each integer is between −1,000,000,000 and 1,000,000,000. Rows may be empty or have different lengths.
Examples
Input:
values = [
[1, 2],
[],
[3, 4]
]
Expected output:
expected = [1, 2, 3, 4]Input:
std::vector<std::vector<long long>> values = {
{1, 2},
{},
{3, 4}
};
Expected output:
std::vector<long long> expected = {1, 2, 3, 4};Input:
long[][] values = new long[][] {
new long[] {1, 2},
new long[] {},
new long[] {3, 4}
};
Expected output:
long[] expected = new long[] {1, 2, 3, 4};Explanation
Read 1 and 2 from the first row. Skip the empty second row. Append 3 and 4 from the last row.
Input:
values = [
[],
[]
]
Expected output:
expected = []Input:
std::vector<std::vector<long long>> values = {
{},
{}
};
Expected output:
std::vector<long long> expected = {};Input:
long[][] values = new long[][] {
new long[] {},
new long[] {}
};
Expected output:
long[] expected = new long[] {};Explanation
Both rows are empty, so there are no integers to append.
How to approach it
Try on your own first, then open one hint at a time.
Hint 1
Traverse the rows in order and append each integer you encounter.
Hint 2
Empty rows contribute nothing, but preserve the order of all values from nonempty rows.
Hint 3
The input is only two levels deep, so no recursive flattening is needed.
Expected complexity
O(n) time, O(n) extra space
References
This challenge is adapted for CodeUtility. Follow the statement and constraints above.