2. Question 2

For a string s which consists only of characters '0' and '1', find the number of subsequences of length 5 which are palindromes.

As the answer can be really big, return the answer mod (10^9 + 7).

Note:

• A palindrome is a sequence that reads the same backward as forward.
• A subsequence is a sequence that can be derived from the given sequence by deleting zero or more elements without changing the order of the remaining elements.
• Two subsequences are considered different if the string that forms the subsequences are different.

Example
s = "0100110"

Using 1-based indexing, the 5 subsequences are:

• Indices (1, 2, 3, 6, 7) -> "01010"
• Indices (1, 2, 3, 5, 7) -> "01010"
• Indices (1, 2, 4, 6, 7) -> "01010"
• Indices (1, 2, 4, 5, 7) -> "01010"
• Indices (1, 2, 5, 6, 7) -> "01110"

5 modulo (10^9 + 7) = 5.

Function Description
Complete the function getPalindromesCount in the editor below.

getPalindromesCount has the following parameter:

• string s: the binary string

Returns

• int: the number of subsequences of length 5 which are palindromes, modulo (10^9 + 7).

Constraints:

• 1 ≤ length of s ≤ 10^5

3. Question 3

Implement a prototype of a friend recommendation system for a social media application.

There are n users indexed from 0 to n-1, and m friends represented as a 2D array, friendships, where the i-th friendship is a connection between users friendships[i][0] and friendships[i][1].

User x is suggested as a friend to user y if x and y are not friends and have the maximum number of common friends, i.e., a friend of both x and y. If there are multiple possible such users x, the one with the minimum index is suggested.

Given n, friendships, and y, determine which user should be recommended to y as a friend. If there is no recommendation available, report -1.

Example
Suppose n = 5, m = 5, and
connections = [[0, 1], [0, 2], [1, 3], [2, 3], [3, 4]].

For user y = 4, the recommendation is user 1.

Function Description
Complete the function friendRecommendation with the following parameters:

• int n: the number of users
• list[list[int]] friendships: a 2D array representing the friendships
• int y: the user to recommend a friend to

Returns

• int: the user index of the friend recommendation, or -1 if no recommendation is available.

Constraints

• 1 ≤ n ≤ 10^5
• 1 ≤ m ≤ 2 × 10^5
• 0 ≤ friendships[i][0], friendships[i][1] < n
• friendships[i][0] ≠ friendships[i][1]

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