add solution: missing-number

Author: dusunax

merge-two-sorted-lists/dusunax.py

@@ -1,7 +1,7 @@
 '''
 # 21. Merge Two Sorted Lists
 
-A. iterative approach: use a temp node to store the result.
+A. iterative approach: use a two pointers to merge the two lists.
 B. recursive approach: use recursion to merge the two lists.
 
 

missing-number/dusunax.py

@@ -0,0 +1,65 @@
+'''
+# 268. Missing Number
+
+A. iterative approach: sort the array and find the missing number.
+B. XOR approach: use XOR to find the missing number.
+  - a ^ a = 0, a ^ 0 = a
+
+## Time and Space Complexity
+
+### A. Iterative Approach
+
+```
+TC: O(n)
+SC: O(1)
+```
+
+#### TC is O(n):
+- sorting the array. = O(n log n)
+- iterating through the array just once to find the missing number. = O(n)
+
+#### SC is O(1):
+- no extra space is used. = O(1)
+
+### B. XOR Approach
+
+```
+TC: O(n)
+SC: O(1)
+```
+
+#### TC is O(n):
+- iterating through the array just once to find the missing number. = O(n)
+
+#### SC is O(1):
+- no extra space is used. = O(1)
+
+'''
+class Solution:
+    '''
+    A. Iterative Approach
+    '''
+    def missingNumberIterative(self, nums: List[int]) -> int:
+        nums.sort()
+        n = len(nums)
+        
+        for i in range(n):
+            if nums[i] != i:
+                return i
+        return n
+    
+    '''
+    B. XOR Approach
+    '''
+    def missingNumberXOR(self, nums: List[int]) -> int:
+        n = len(nums)
+        xor_all = 0
+        xor_nums = 0
+
+        for i in range(n + 1):
+            xor_all ^= i
+        
+        for num in nums:
+            xor_nums ^= num
+
+        return xor_all^xor_nums