Generate Parentheses

Problem

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:
"((()))", "(()())", "(())()", "()(())", "()()()"

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Idea

dfs暴力搜索，解的长度为2n，当找到2n长的一个solution后判断其是否valid

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Solution

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