Longest Valid Parentheses

Problem

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.

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Idea

一个stack记录所有没有匹配的{的位置

lastPos 记录上个}的位置.

碰到{就把位置插到stack，碰到}就先Pop,然后看看stack是否空，如果空则表示1个{对应1个},则length = i-lastPos。 如果stack不空,则length = i-lefts.top()

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Solution

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