When no more interesting kata can be resolved, I just choose to create the new kata, to solve their own, to enjoy the process --myjinxin2015 said

Description:

Given an unsorted array arr that contains some positive integers. It may be one of the following:

1. There are numbers 1 to n, only one number is 
   duplicate(repeated two times), the other numbers are unique. 
   That is, there are n+1 elements in the array. 
   e.g. [1,2,3,6,5,4,1]

2. There are numbers 1 to n, only one number is 
   unique, the other numbers are repeated two times. 
   That is, there are 2*n-1 elements in the array. 
   e.g. [1,2,3,1,2,3,4]

Your task is to determine the type of the array, if it is the first type, to return the duplicate; if it is second type, return the unique.

Note:

• All numbers are positive integers that from 1 to n;
• The length of array always more than 5;
• Please pay attention to optimizing the code to avoid time out.

Some Examples

input                                output
[1,2,3,6,5,4,1]                      1
[1,2,3,1,2,3,4]                      4
[3,6,9,2,5,8,1,4,8,7]                8
[9,8,7,1,2,3,9,7,1,2,3,4,4,5,5,6,6]  8