start
ba是回文串么？不是len = 0beginIndex = 0endIndex = 0
end
a
b
bab是回文串么？不是len = 3beginIndex = start = 0endIndex = end = 2
bab是回文串么？是len1 = 3 = end - start + 1 = 2 - 0 + 1len1 > lenlen = len1beginIndex = start = 0endIndex = end = 2
e
d
暴力解法
第一轮：
bab是回文串么？是len1 = end - start +1 = 3 -1 + 1 = 3len1 > lenbeginIndex = start = 0endIndex = end = 2
第二轮：
