Merge pull request #1911 from sonjh1217/main

[sonjh1217] WEEK 09 solutions

Author: sonjh1217

linked-list-cycle/sonjh1217.swift

@@ -0,0 +1,63 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.next = nil
+ *     }
+ * }
+ */
+
+extension ListNode: Equatable {
+    public static func == (lhs: ListNode, rhs: ListNode) -> Bool {
+        return lhs === rhs
+    }
+}
+
+extension ListNode: Hashable {
+    public func hash(into hasher: inout Hasher) {
+        hasher.combine(ObjectIdentifier(self))
+    }
+}
+
+class Solution {
+    // O(n) time / O(n) space
+    func hasCycle(_ head: ListNode?) -> Bool {
+        var visitedNodes = Set<ListNode>()
+        var node = head
+        
+        while node != nil {
+            guard let currentNode = node else {
+                return false
+            }
+            
+            if visitedNodes.contains(currentNode) {
+                return true
+            }
+            
+            visitedNodes.insert(currentNode)
+            node = currentNode.next
+        }
+        
+        return false
+    }
+    
+    // O(n) time / O(1) space
+    func hasCycleFloyd(_ head: ListNode?) -> Bool {
+        var slow = head
+        var fast = head
+        
+        while fast != nil && fast?.next != nil {
+            slow = slow?.next
+            fast = fast?.next?.next
+            
+            if slow === fast {
+                return true
+            }
+        }
+        
+        return false
+    }
+}

pacific-atlantic-water-flow/sonjh1217.swift

@@ -0,0 +1,118 @@
+class Solution {
+    // O(m*n) time / O(m*n) space
+    func pacificAtlanticBFS(_ heights: [[Int]]) -> [[Int]] {
+        var result = [[Int]]()
+        guard let first = heights.first else {
+            return result
+        }
+        let m = heights.count
+        let n = first.count
+        
+        var goesPacific = Array(repeating: Array(repeating: false, count: n), count: m)
+        var goesAtlantic = Array(repeating: Array(repeating: false, count: n), count: m)
+        let directions = [(1,0), (-1,0), (0,1), (0,-1)]
+        
+        func bfs(queue: [(Int, Int)], goes: inout [[Bool]]) {
+            var queue = queue
+            var head = 0
+            for (i, j) in queue {
+                goes[i][j] = true
+            }
+            
+            while head < queue.count {
+                let (i, j) = queue[head]
+                head += 1
+                
+                for (di, dj) in directions {
+                    let ni = i + di
+                    let nj = j + dj
+                    if ni >= 0
+                        && ni < m
+                        && nj >= 0
+                        && nj < n
+                        && !goes[ni][nj]
+                        && heights[ni][nj] >= heights[i][j]  {
+                        goes[ni][nj] = true
+                        queue.append((ni, nj))
+                    }
+                }
+            }
+        }
+        
+        var pacificStarts = [(Int, Int)]()
+        var atlanticStarts = [(Int, Int)]()
+        for i in 0..<m {
+            pacificStarts.append((i, 0))
+            atlanticStarts.append((i, n-1))
+        }
+        
+        for j in 0..<n {
+            pacificStarts.append((0, j))
+            atlanticStarts.append((m-1, j))
+        }
+        
+        bfs(queue: pacificStarts, goes: &goesPacific)
+        bfs(queue: atlanticStarts, goes: &goesAtlantic)
+        
+        for i in 0..<m {
+            for j in 0..<n {
+                if goesPacific[i][j] && goesAtlantic[i][j] {
+                    result.append([i, j])
+                }
+            }
+        }
+        return result
+    }
+    
+    // O(m*n) time / O(m*n) space
+    func pacificAtlanticDFS(_ heights: [[Int]]) -> [[Int]] {
+        var result = [[Int]]()
+        guard let first = heights.first else {
+            return result
+        }
+        let m = heights.count
+        let n = first.count
+        
+        var goesPacific = Array(repeating: Array(repeating: false, count: n), count: m)
+        var goesAtlantic = Array(repeating: Array(repeating: false, count: n), count: m)
+        let directions = [(1,0), (-1,0), (0,1), (0,-1)]
+        
+        func dfs(i: Int, j: Int, goes: inout [[Bool]]) {
+            if goes[i][j] {
+                return
+            }
+            goes[i][j] = true
+            
+            for (di, dj) in directions {
+                let ni = i + di
+                let nj = j + dj
+                if ni >= 0
+                    && ni < m
+                    && nj >= 0
+                    && nj < n
+                    && heights[ni][nj] >= heights[i][j]  {
+                    dfs(i: ni, j: nj, goes: &goes)
+                }
+            }
+        }
+        
+        for i in 0..<m {
+            dfs(i: i, j: 0, goes: &goesPacific)
+            dfs(i: i, j: n-1, goes: &goesAtlantic)
+        }
+        
+        for j in 0..<n {
+            dfs(i: 0, j: j, goes: &goesPacific )
+            dfs(i: m-1, j: j, goes: &goesAtlantic )
+        }
+        
+        for i in 0..<m {
+            for j in 0..<n {
+                if goesPacific[i][j] && goesAtlantic[i][j] {
+                    result.append([i, j])
+                }
+            }
+        }
+        return result
+    }
+}