22. Generate Parentheses

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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:

[  "((()))",  "(()())",  "(())()",  "()(())",  "()()()"]

这类题目基本都可以通过递归调用解决。

class Solution {    public void handler(List<String> lst, StringBuilder str, int l, int r, int n){        if (str.length() == 2*n){            lst.add(new String(str));            return;        }        if (l < n){            str.append('(');            handler(lst, str, l+1, r, n);            str.deleteCharAt(str.length() - 1);        }        if (l > r){            str.append(')');            handler(lst, str, l, r+1, n);            str.deleteCharAt(str.length() - 1);        }    }    public List<String> generateParenthesis(int n) {        List<String> lst = new ArrayList<>();        StringBuilder str = new StringBuilder();        handler(lst, str, 0, 0, n);        return lst;    }}

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