Longest Valid Parentheses
Given a string containing just the characters '(' and ')',
find the length of the longest valid (well-formed) parentheses substring.
For "(()", the longest valid parentheses substring is "()",
which has length = 2.
Another example is ")()())", where the longest valid parentheses substring
is "()()", which has length = 4.
For loop：本题从左到右扫描一次（类似于排列组合里的楼梯）并计录当前段。扫描过程中左括号时刻不能比右括号少，否则重新计数。此外当左右括号相等时为一个有效括号序列，可以用来更新maxLen。
从右到左扫描的时候反之。