You are given an array of strings arr. Return the maximum possible length of a string s formed by concatenating a subsequence of arr such that all characters in s are unique.
You are given two strings s1 and s2 of equal length consisting only of ‘x’ and ‘y’. Your task is to make these two strings equal by swapping characters between the two strings. Return the minimum number of swaps required to make s1 and s2 equal, or return -1 if it is impossible.
You are given an array of integers nums and an integer k. A subarray is considered ’nice’ if it contains exactly k odd numbers. Your task is to return the number of nice subarrays in the given array.
You are given a string s containing lowercase English letters and parentheses. Your task is to remove the minimum number of parentheses to make the string valid.
You are given a binary matrix with 2 rows and n columns, where each element is either 0 or 1. The sum of elements in the 0-th (upper) row is given by upper, the sum of elements in the 1-st (lower) row is given by lower, and the column-wise sum is given by colsum. Your task is to reconstruct the matrix based on these sums. If reconstruction is not possible, return an empty matrix.
You are given a 2D grid with 0s (land) and 1s (water). An island is a group of 0s connected 4-directionally, and a closed island is a group of 0s completely surrounded by 1s. Your task is to count how many closed islands are present in the grid.