《leetCode》：Generate Parentheses（hard）

题目：

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:

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

public List<String> generateParenthesis(int n) {
List<String> list = new ArrayList<String>();
backtrack(list, "", 0, 0, n);
return list;
}

public void backtrack(List<String> list, String str, int open, int close, int max){

if(str.length() == max*2){
list.add(str);
return;
}

if(open < max)
backtrack(list, str+"(", open+1, close, max);
if(close < open)
backtrack(list, str+")", open, close+1, max);
}