BFS Daily Coding Challenge 55

DailyCodingChallenge55.swift

/*
You are given an M by N matrix consisting of booleans that represents a board. Each True boolean represents a wall. Each False boolean represents a tile you can walk on.

Given this matrix, a start coordinate, and an end coordinate, return the minimum number of steps required to reach the end coordinate from the start. If there is no possible path, then return null. You can move up, left, down, and right. You cannot move through walls. You cannot wrap around the edges of the board.

For example, given the following board:

[[f, f, f, f],
[t, t, f, t],
[f, f, f, f],
[f, f, f, f]]

and start = (3, 0) (bottom left) and end = (0, 0) (top left), the minimum number of steps required to reach the end is 7, since we would need to go through (1, 2) because there is a wall everywhere else on the second row.
*/

/*

modified BFS, where we store the current number of steps along witht he current node


visited[board.size][board.size]
queue

queue.enquqeue(PositionLength(start, 0)))

while !queue.isEmpty
  let cur = queue.dequeue()
  visited[cur.position] = true
  if cur.position = end {
    return cur.length
  }
  let neighbors: [PosiitonLneght] = getNeighbors(cur, baord)
  for neghbor in nehgbors
    if !visited[neighbor.position]
      queue.enqueue(neighbor)
      
return nil


O(V*E) = O(n^2) run time

O(V+E) = O(n^2) space for the visited array

*/

struct PositionLength {
  var position: Coordinate
  var length: Int
}

struct Coordinate: Equatable {
  var row: Int
  var col: Int
}

func minSteps(board: [[Bool]], start: Coordinate, end: Coordinate) -> Int? {
  var visited: [[Bool]] = Array(repeating: Array(repeating: false, count: board.count), count: board.count)
  
  var queue: [PositionLength] = []
  queue.append(PositionLength(position: start, length: 0))
  while !queue.isEmpty {
    let cur: PositionLength = queue.removeFirst()
    visited[cur.position.row][cur.position.col] = true
    if cur.position == end {
      return cur.length
    }
    let neighbors: [PositionLength] = getNeighbors(position:cur, board: board)
    for neighbor in neighbors {
      if !visited[neighbor.position.row][neighbor.position.col] {
        queue.append(neighbor)
      }
    }
  }
  return nil
}

func getNeighbors(position: PositionLength, board: [[Bool]]) -> [PositionLength] {
  // valid neighbor not outof bounds
  // valid neighbor in board has a value of false
  // right
  
  var neighbors: [PositionLength] = []
  
  let curCol = position.position.col
  let curRow = position.position.row
  let curLen = position.length
  
  // Right
  let right = curCol + 1
  if right < board.count && board[curRow][right] != true {
    let temp = Coordinate(row:curRow, col:right)
    neighbors.append(PositionLength(position: temp, length: curLen+1))
  }
    
  // Bottom
  let bottom = curRow + 1
  if bottom < board.count && board[bottom][curCol] != true {
    let temp = Coordinate(row:bottom, col:curCol)
    neighbors.append(PositionLength(position: temp, length: curLen+1))
  }
  
  // Top
  let top = curRow - 1
  if top >= 0 && board[top][curCol] != true {
    let temp = Coordinate(row:top, col:curCol)
    neighbors.append(PositionLength(position: temp, length: curLen+1))
  }
  
  // Left
  let left = curCol - 1
  if left >= 0 && board[curRow][left] != true {
    let temp = Coordinate(row:curRow, col:left)
    neighbors.append(PositionLength(position: temp, length: curLen+1))
  }
  
  return neighbors
}

let board: [[Bool]] = [
  [false, false, false, false],
  [true, true, true, false],
  [false, false, false, false],
  [false, false, false, false]
]

let start = Coordinate(row: 2, col: 0)

let end = Coordinate(row: board.count-1, col: board.count-1)

print("\(minSteps(board:board, start:start, end:end) ?? -1)")