22.Generate Parentheses

给出n对括号，输出所有正确的组合方式。

如n=3,输出:

[

“((()))”,

“(()())”,

“(())()”,

“()(())”,

“()()()”

]

def generate_parentheses(left, right, out=None, res=None):
if left > right:
return
if out is None:
out = ‘’
if res is None:
res = []

if left == 0 and right == 0:  
    res.append(out)  
else:  
    if left > 0:  
        generate_parentheses(left-1, right, out+'(', res)  
    if right > 0 :  
        generate_parentheses(left, right-1, out+')', res)  
return res

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