add: Add other solutions

Author: WhiteHyun

missing-number/WhiteHyun.swift

@@ -10,4 +10,21 @@ final class Solution {
   func missingNumber(_ nums: [Int]) -> Int {
     (0 ... nums.count).reduce(0, +) - nums.reduce(0, +)
   }
+
+  func missingNumber2(_ nums: [Int]) -> Int {
+    nums.count * (nums.count + 1) / 2 - nums.reduce(0, +)
+  }
+
+  func missingNumber3(_ nums: [Int]) -> Int {
+    Set(0...nums.count).subtracting(Set(nums)).first!
+  }
+
+  func missingNumber4(_ nums: [Int]) -> Int {
+    var answer = 0
+    for i in 0 ..< nums.count {
+      answer = answer ^ i ^ nums[i]
+    }
+
+    return answer ^ nums.count
+  }
 }

number-of-1-bits/WhiteHyun.swift

@@ -10,4 +10,15 @@ final class Solution {
   func hammingWeight(_ n: Int) -> Int {
     n.nonzeroBitCount
   }
+  
+  func hammingWeight2(_ n: Int) -> Int {
+    var number = n
+    var answer = 0
+    while number != 0 {
+      answer += number & 1
+      number >>= 1
+    }
+
+    return answer
+  }
 }