[Math] Add a solution to K-th Smallest in Lexicographical Order

Author: Yi Gu

Diff for: Math/KthSmallestLexicographicalOrder.swift

@@ -0,0 +1,34 @@
+/**
+ * Question Link: https://leetcode.com/problems/k-th-smallest-in-lexicographical-order/
+ * Primary idea: Image it is a ten-deminsion tree, we only need to calculate the number 
+ *               of children for each node, and keep minus until we get the right one
+ * 
+ * Note: Swift does not have a way to access a character in a string with O(1), 
+ *       thus we have to first transfer the string to a character array
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class KthSmallestLexicographicalOrder {
+    func findKthNumber(_ n: Int, _ k: Int) -> Int {
+        var curt = 1, k = k - 1
+        
+        while k > 0 {
+            var step = 0, first = curt, last = curt + 1
+            while first <= n {
+                step += min(n + 1, last) - first
+                first *= 10
+                last *= 10
+            }
+            if step <= k {
+                curt += 1
+                k -= step
+            } else {
+                curt *= 10
+                k -= 1
+            }
+        }
+        
+        return curt
+    }
+}