The Severn Estuary is home to huge numbers of sprats and salmon, and the twaite shad, a protected migrating species which spawns in the tributaries of the River Severn.
方向遍历顺序说明下一个倒序（从右往左）先处理右侧，栈里存「右侧候选」上一个正序（从左往右）先处理左侧，栈里存「左侧候选」
。关于这个话题，WPS下载最新地址提供了深入分析
float r_sequence(int x, int y)
except Exception: