[Array] Add a solution to Find All Numbers Disappeared in an Array

Array/FindDisappearedNumbers.swift

@@ -0,0 +1,30 @@
+/**
+ * Question Link: https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/
+ * primary idea: Traverse the array and get num really position in array, then set negative.
+ *               In the final, filter greater than 0 num.
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class FindDisappearedNumbers {
+  func findDisappearedNumbers(_ nums: [Int]) -> [Int] {
+    var nums = nums
+    var result = [Int]()
+
+    for i in 0..<nums.count {
+      let index = abs(nums[i]) - 1
+      if nums[index] > 0 {
+        nums[index] = -nums[index]
+      }
+    }
+
+    for i in 0..<nums.count {
+      if nums[i] > 0 {
+        result.append(i+1)
+      }
+    }
+
+    return result
+  }
+}