说明：  关于Recaman s序列的问题，一个简单解决方法，保证时间和空间的限制。
(The Recaman s sequence is defined by a0 = 0 for m > 0, am = am-1- m if the resulting am is positive and not already in the sequence, otherwise am = am-1+ m. The first few numbers in the Recaman s Sequence is 0, 1, 3, 6, 2, 7, 13, 20, 12, 21, 11, 22, 10, 23, 9... . Given k, your task is to calculate ak. )