leetcode-286-walls-and-gates.swift

/**

Copyright (c) 2020 David Seek, LLC

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https://leetcode.com/problems/walls-and-gates/


You are given a m x n 2D grid initialized with these three possible values.

-1 - A wall or an obstacle.
0 - A gate.
INF - Infinity means an empty room. We use the value 231 - 1 = 2147483647 to represent INF as you may assume that the distance to a gate is less than 2147483647.
Fill each empty room with the distance to its nearest gate. If it is impossible to reach a gate, it should be filled with INF.

Example: 

Given the 2D grid:

INF  -1  0  INF
INF INF INF  -1
INF  -1 INF  -1
  0  -1 INF INF
After running your function, the 2D grid should be:

  3  -1   0   1
  2   2   1  -1
  1  -1   2  -1
  0  -1   3   4

*/

/**
Big O Annotation
Time complexity O(n^2) where n is the amount of elements in nums.
Space complexity O(1) we're not using any extra space.
*/
func wallsAndGates(_ rooms: inout [[Int]]) {
        
    // Iterate through every row
    for (i, row) in rooms.enumerated() {
        
        // And every column
        for (j, column) in row.enumerated() {
            
            // Check if we have a gate
            if column == 0 {

                // And run our DFS function
                setRooms(&rooms, atRow: i, andColumn: j)
            }
            
        }
    }
}

// DFS
private func setRooms(_ rooms: inout [[Int]], atRow row: Int, andColumn column: Int, to value: Int = 0) {
    
    // Check left, right, upper and lower bounds
    guard row >= 0 && column >= 0 && row < rooms.count && column < rooms[row].count else {
        return 
    }
    
    /**
    We need to check the room value.
    As we only want to update rooms that are 
    larger than our value.

    That leaves out the doors with value -1
    as the minimum value is 0.

    And it sets all the Int.max values to at least 0+1
    */
    guard value <= rooms[row][column] else {
        return
    }
    
    // Set the room to value
    rooms[row][column] = value
    
    // And traverse in all directions, updating value by 1
    setRooms(&rooms, atRow: row + 1, andColumn: column, to: value + 1)
    setRooms(&rooms, atRow: row - 1, andColumn: column, to: value + 1)
    setRooms(&rooms, atRow: row, andColumn: column + 1, to: value + 1)
    setRooms(&rooms, atRow: row, andColumn: column - 1, to: value + 1)
}