一道leetcode上没有，看似简单，写起来不简单的题目

Given a list of unique integers: eg, [3,5,10,5,7,2]:

1) Skip every 3rd number
2) Then output the rest in a hashtable: {item N: item N+1}

So
[3,6,10,5,7,2] becomes: {3:6, 5:7}
[3,6,10,5,7,2,1] becomes: {3:6, 5:7, 1:Null}

判断一下index就可以了

双指针？