[String] Add a solution to Find the Closest Palindrome

DP/BestTimeBuySellStockIV.swift

@@ -21,7 +21,11 @@
             return makeMaxProfit(prices)
         }
 
-        var local = Array(repeating: 0, count: k + 1), global = Array(repeating: 0, count: k + 1)
+        // local[i] means the maxProfit when sell happens at ith day
+        var local = Array(repeating: 0, count: k + 1)
+
+        // global[i] means the maxProfit at ith day
+        var global = Array(repeating: 0, count: k + 1)
 
         for i in 0..<n - 1 {
             let diff = prices[i + 1] - prices[i]

README.md

@@ -4,7 +4,7 @@
 ![Leetcode](./logo.png?style=centerme)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 292 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
+[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 293 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -130,6 +130,7 @@
 [Longest Substring with At Most Two Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/)| [Swift](./String/LongestSubstringMostTwoDistinctCharacters.swift)| Hard| O(n)| O(n)|
 [Longest Substring with At Most K Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/)| [Swift](./String/LongestSubstringMostKDistinctCharacters.swift)| Hard| O(n)| O(n)|
 [Text Justification](https://leetcode.com/problems/text-justification/)| [Swift](./String/TextJustification.swift)| Hard| O(n)| O(n)|
+[Find the Closest Palindrome](https://leetcode.com/problems/find-the-closest-palindrome/)| [Swift](./String/FindClosestPalindrome.swift)| Hard| O(n)| O(n)|
 
 ## Linked List
 | Title | Solution | Difficulty | Time | Space |

String/FindClosestPalindrome.swift

@@ -0,0 +1,69 @@
+/**
+ * Question Link: https://leetcode.com/problems/find-the-closest-palindrome/
+ * Primary idea: Five possible cases -- the numbers with digits less or greater than current number; 
+ * 				 the numbers with same digits' number but replication of the mutation
+ * 				 (+1 or -1 if the original number itself is palindrome) of the first half.
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+ class FindClosestPalindrome {
+    func nearestPalindromic(_ n: String) -> String {
+        let chars = Array(n)
+        var candidates = Set<String>()
+
+        candidates.insert(buildLeftNum(n))
+        candidates.insert(buildRightNum(n))
+
+        for distance in -1...1 {
+            candidates.insert(buildPalindromeOnMiddle(chars, distance))
+        }
+
+        return findNearest(n, candidates)  
+    }
+
+    private func buildLeftNum(_ num: String) -> String {        
+        return String(repeating: "9", count: num.count - 1)
+    }
+
+    private func buildRightNum(_ num: String) -> String {
+        return "1" + String(repeating: "0", count: num.count - 1) + "1"
+    }
+
+    private func buildPalindromeOnMiddle(_ n: [Character], _ distance: Int) -> String {
+        let middleIdx = n.count % 2 == 0 ? n.count / 2 - 1: n.count / 2
+        let leftPart = String(Int(String(n[0...middleIdx]))! + distance)
+
+        if n.count % 2 != 0 {
+            return String(leftPart.dropLast() + leftPart.reversed())
+        } else {
+            return String(leftPart + leftPart.reversed())
+        }
+    }
+
+    private func findNearest(_ n: String, _ candidates: Set<String>) -> String {
+        guard let num = Int(n) else {
+            fatalError("Invalid Input")
+        }
+
+        var nearest = 0
+
+        for candidate in candidates {
+            guard let cNum = Int(candidate) else {
+                continue
+            }
+            if cNum == num {
+                continue
+            }
+
+            if abs(cNum - num) < abs(nearest - num) {
+                nearest = cNum
+            } else if abs(cNum - num) == abs(nearest - num) {
+                nearest = min(cNum, nearest)
+            }
+        }
+
+        return String(nearest)
+    }
+}