Merge pull request soapyigu#243 from WAMaker/master

[Math] Add a solution to Permutation Sequence

Author: soapyigu

Math/PermutationSequence.swift

@@ -0,0 +1,39 @@
+/**
+ * Question Link: https://leetcode.com/problems/permutation-sequence/
+ * Primary idea: Iterate and change the array from last to the first
+ *
+ * Time Complexity: O(n^2), Space Complexity: O(1)
+ */
+
+class PermutationSequence {
+    func getPermutation(_ n: Int, _ k: Int) -> String {
+        var numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+        var factorial = 1
+        for i in 1 ..< n {
+            factorial *= i
+        }
+
+        var result = ""
+        var k = k
+        var divisor = n - 1
+
+        for i in 0 ..< n {
+            for (index, number) in numbers.enumerated() {
+                if k > factorial {
+                    k -= factorial
+                } else {
+                    result += "\(number)"
+                    numbers.remove(at: index)
+                    break
+                }
+            }
+            if divisor > 1 {
+                factorial /= divisor
+                divisor -= 1
+            }
+        }
+
+        return result
+    }
+}

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 283 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 284 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -320,6 +320,7 @@
 [Counting Bits](https://leetcode.com/problems/counting-bits/)| [Swift](./Math/CountingBits.swift)| Medium| O(n)| O(n)|
 [K-th Smallest in Lexicographical Order](https://leetcode.com/problems/k-th-smallest-in-lexicographical-order/)| [Swift](./Math/KthSmallestLexicographicalOrder.swift)| Hard| O(n)| O(1)|
 [Gary Code](https://leetcode.com/problems/gray-code/)| [Swift](./Math/GaryCode.swift)| Medium| O(n)| O(2^n)|
+[Permutation Sequence](https://leetcode.com/problems/permutation-sequence/)| [Swift](./Math/PermutationSequence.swift)| Medium| O(n^2)| O(1)|
 
 ## Search
 | Title | Solution | Difficulty | Time | Space |
@@ -801,7 +802,7 @@
 |	[Swift](./DP/UniquePathsII.swift)	|	63	|	[Unique Paths II](https://oj.leetcode.com/problems/unique-paths-ii/)	| Medium	|
 |	[Swift](./DP/UniquePaths.swift)	|	62	|	[Unique Paths](https://oj.leetcode.com/problems/unique-paths/)	| Medium	|
 |	[Swift](./LinkedList/RotateList.swift)	|	61	|	[Rotate List](https://oj.leetcode.com/problems/rotate-list/)	| Medium	|
-|		|	60	|	[Permutation Sequence](https://oj.leetcode.com/problems/permutation-sequence/)	| Medium	|
+|	[Swift](./Math/PermutationSequence.swift)	|	60	|	[Permutation Sequence](https://oj.leetcode.com/problems/permutation-sequence/)	| Medium	|
 |	[Swift](./Array/SpiralMatrixII.swift)	|	59	|	[Spiral Matrix II](https://oj.leetcode.com/problems/spiral-matrix-ii/)	| Medium	|
 |	[Swift](./String/LengthLastWord.swift)	|	58	|	[Length of Last Word](https://oj.leetcode.com/problems/length-of-last-word/)	| Easy	|
 | [Swift](./Sort/InsertInterval.swift) |	57	|	[Insert Interval](https://oj.leetcode.com/problems/insert-interval/)	| Hard	|