[sonjh1217] WEEK 08 solutions

clone-graph/sonjh1217.swift

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+/**
+ * Definition for a Node.
+ * public class Node {
+ *     public var val: Int
+ *     public var neighbors: [Node?]
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.neighbors = []
+ *     }
+ * }
+ */
+
+class Solution {
+    // O(V+E) time / O(V) space
+    func cloneGraph(_ node: Node?) -> Node? {
+        guard let node = node else {
+            return nil
+        }
+        var newNodeByVal = [Int: Node]()
+        return copy(node: node, newNodeByVal: &newNodeByVal)
+    }
+
+    func copy(node: Node, newNodeByVal: inout [Int: Node]) -> Node {
+        if let node = newNodeByVal[node.val] {
+            return node
+        }
+        var newNode = Node(node.val)
+        newNodeByVal[node.val] = newNode
+
+        for neighbor in node.neighbors {
+            guard let neighbor = neighbor else {
+                continue
+            }
+            let newNeighbor = copy(node: neighbor, newNodeByVal: &newNodeByVal)
+            newNode.neighbors.append(newNeighbor)
+        }
+        return newNode
+    }
+}

longest-repeating-character-replacement/sonjh1217.swift

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+class Solution {
+    // O(n) time / O(n) space
+    func characterReplacement(_ s: String, _ k: Int) -> Int {
+        var windowStart = 0
+        var maxLength = 0
+        var maxCharacterRepeat = 0
+        var countByCharacter = [Character: Int]()
+        var characters = Array(s)
+
+        for windowEnd in 0..<characters.count {
+            countByCharacter[characters[windowEnd], default: 0] += 1
+            maxCharacterRepeat = max(maxCharacterRepeat, countByCharacter[characters[windowEnd]]!)
+
+            var length = windowEnd - windowStart + 1
+            if length - maxCharacterRepeat <= k {
+                maxLength = max(maxLength, length)
+            } else {
+                while (windowEnd - windowStart + 1) - maxCharacterRepeat > k {
+                    countByCharacter[characters[windowStart]]! -= 1
+                    windowStart += 1
+                }
+            }
+        }
+        return maxLength
+    }
+}

palindromic-substrings/sonjh1217.swift

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+class Solution {
+    //dynamic programming
+    // O(n^2) time / O(n^2) space
+    func countSubstrings1(_ s: String) -> Int {
+        var count = 0
+        var characters = Array(s)
+        var isPalindromes = Array(repeating: Array(repeating: false, count: characters.count), count: characters.count)
+
+        for i in 0..<characters.count {
+            isPalindromes[i][i] = true
+            count += 1
+        }
+
+        for right in 1..<characters.count {
+            for left in 0..<right {
+                if characters[left] == characters[right]
+                    && (isPalindromes[left+1][right-1] || right-left <= 2) {
+                    isPalindromes[left][right] = true
+                    count += 1
+                }
+            }
+        }
+
+        return count
+    }
+
+    //two pointers
+    // O(n^2) time / O(n) space
+    func countSubstrings2(_ s: String) -> Int {
+        var count = 0
+        var characters = Array(s)
+        var left: Int
+        var right: Int
+
+        for i in 0..<characters.count{
+            left = i
+            right = i
+
+            while left >= 0
+                    && right < characters.count
+                    && characters[left] == characters[right] {
+                count += 1
+
+                left -= 1
+                right += 1
+            }
+
+            left = i
+            right = i+1
+
+            while left >= 0
+                    && right < characters.count
+                    && characters[left] == characters[right] {
+                count += 1
+
+                left -= 1
+                right += 1
+            }
+        }
+
+        return count
+    }
+
+}
+

reverse-bits/sonjh1217.swift

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+class Solution {
+    // O(1) time / O(1) space
+    func reverseBits(_ n: Int) -> Int {
+        var number = n
+        var reversed = 0
+
+        for _ in 0..<32 {
+            reversed <<= 1
+            reversed |= (number & 1)
+            number >>= 1
+        }
+        return reversed
+    }
+}