Time: 11min
Method: binary search
Reflection: need to break from while look when the target is found
Time complexity: O(logN)
Space Complexity: O(1)
Time: 10min
Method: brute force
Reflection: index boundary; while vs for loop
Time complexity: O(N^2)
Space complexity: O(1)
Time: 10min
Method: two pointers - fast vs slow
Reflection: left pointers record the correct elements. since left pointer always slower than right pointer, they can be swapped in place.
Time complexity: O(N)
Space complexity: O(1)
