[BFS] Add a solution to Shortest Distance from All Buildings

Author: soapyigu

BFS/ShortestDistanceAllBuildings.swift

@@ -0,0 +1,74 @@
+/**
+ * Question Link: https://leetcode.com/problems/shortest-distance-from-all-buildings
+ * Primary idea: Use BFS to caculate distance from building to all valid points, and 
+ *               then figuare out the smallest one of all candidates
+ * 
+ * Time Complexity: O((mn)^2), Space Complexity: O(mn)
+ *
+ */
+
+class ShortestDistanceAllBuildings {
+    func shortestDistance(_ grid: [[Int]]) -> Int {
+        guard grid.count > 0 && grid[0].count > 0 else {
+            return -1
+        }
+        
+        let m = grid.count, n = grid[0].count
+        var distances = Array(repeating: Array(repeating: 0, count: n), count: m), reachableNums = Array(repeating: Array(repeating: 0, count: n), count: m)
+        var shortestDistance = Int.max, buildingNums = 0
+        
+        for i in 0..<m {
+            for j in 0..<n {
+                if grid[i][j] == 1 {
+                    buildingNums += 1
+                    bfs(grid, &distances, &reachableNums, i, j)
+                }
+            }
+        }
+        
+       for i in 0..<m {
+            for j in 0..<n {
+                if reachableNums[i][j] == buildingNums {
+                    shortestDistance = min(shortestDistance, distances[i][j])
+                }
+            }
+        }
+        
+        return shortestDistance == Int.max ? -1 : shortestDistance
+    }
+    
+    fileprivate func bfs(_ grid: [[Int]], _ distances: inout [[Int]], _ reachableNums: inout [[Int]], _ i: Int, _ j: Int) {
+        let m = grid.count, n = grid[0].count
+        var pointsQueue = [(i, j)], distancesQueue = [0]
+        var visited = Array(repeating: Array(repeating: false, count: n), count: m)
+        
+        while !pointsQueue.isEmpty {
+            let currentPoint = pointsQueue.removeFirst(), x = currentPoint.0, y = currentPoint.1
+            let currentDistance = distancesQueue.removeFirst()
+            let ranges = [(x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)]
+            
+            // set distances and reachableNums
+            if grid[x][y] == 0 {
+                distances[x][y] += currentDistance
+                reachableNums[x][y] += 1
+            } 
+            
+            // search neighbors
+            for range in ranges {
+                let xx = range.0, yy = range.1
+                
+                guard xx >= 0 && xx < m && yy >= 0 && yy < n && !visited[xx][yy] else {
+                    continue
+                }
+                
+                guard grid[xx][yy] == 0 else {
+                    continue
+                }
+                
+                visited[xx][yy] = true
+                pointsQueue.append((xx, yy))
+                distancesQueue.append(currentDistance + 1)
+            }
+        }
+    }
+}