提取子串的算法示例 pos=2, len=3 pos=5, len=4 nfinity infinity 超 posten -1 posten -1 curLen-l ≥ curlen提取子串的算法示例 pos+len -1 pos+len -1  curLen-1  curLen i n f i n i t y i n f i n i t y pos = 2, len = 3 pos = 5, len = 4 f i n i t y 超出