Leetcode Problem 351. Android Unlock Patterns

351. Android Unlock Patterns

Leetcode Solutions

Initialize a 2D array skip to represent the middle dot that needs to be visited if a move is made between two non-adjacent dots.
Create a boolean array visited to keep track of which dots have been used in the current pattern.
Define a recursive function dfs that takes the current dot, the remaining number of dots to be used, and the visited array as arguments.
In dfs, if the number of dots to be used is zero, return 1 as a valid pattern is found.
Otherwise, iterate over all possible next dots, and for each: a. Check if the next dot is not visited and either adjacent to the current dot or the skip dot between the current and next dot is already visited. b. If valid, mark the next dot as visited and recursively call dfs with the next dot and decreased number of dots to be used. c. Backtrack by marking the next dot as not visited.
Use the dfs function to count patterns starting from each dot, taking into account symmetry to avoid redundant calculations.
Sum up the counts for each starting dot to get the total number of valid patterns.

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