Add a solution to Next Palindrome Using Same Digits

Author: soapyigu

Array/NextPalindromeUsingSameDigits.swift

@@ -0,0 +1,56 @@
+/**
+ * Question Link: https://leetcode.com/problems/next-palindrome-using-same-digits/
+ * Primary idea: Figure out the first half's next permutation, then double it to the result.
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class NextPalindromeUsingSameDigits {
+    func nextPalindrome(_ num: String) -> String {
+        let num = Array(num)
+        var firstHalf = num[0..<num.count / 2].compactMap { Int(String($0)) }
+        
+        if num.count <= 1 || !nextPermutation(&firstHalf) {
+            return ""
+        }
+        
+        let firstHalfStr = firstHalf.map { String($0) }.joined()
+        
+        if num.count % 2 == 0 {
+            return firstHalfStr + firstHalfStr.reversed()
+        } else {
+            return firstHalfStr + String(num[num.count / 2]) + firstHalfStr.reversed()
+        }
+    }
+
+    private func nextPermutation(_ nums: inout [Int]) -> Bool {
+        var violateIdx = -1
+        
+        for i in (1..<nums.count).reversed() {
+            if nums[i] > nums[i - 1] {
+                violateIdx = i - 1
+                break
+            }
+        }
+        
+        if violateIdx == -1 {
+            nums.reverse()
+            return false
+        }
+        
+        for i in ((violateIdx + 1)..<nums.count).reversed() {
+            if nums[i] > nums[violateIdx] {
+                swap(&nums, i, violateIdx)
+                break
+            }
+        }
+        
+        nums[(violateIdx + 1)...].reverse()
+        return true
+    }
+
+    private func swap(_ nums: inout [Int], _ l: Int, _ r: Int) {
+        (nums[l], nums[r]) = (nums[r], nums[l])
+    }
+}