letcode：22
Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
public List<String> generateParenthesis(int n) {
List<String> res = new ArrayList<>();
gen(res, n, n, "");
return res;
}
private void gen(List<String> res, int left, int right, String str) {
if (left == 0 && right == 0) {
res.add(str);
return;
}
if (left > 0) {
gen(res, left - 1, right, str + "(");
}
if(right > left){
gen(res, left, right - 1,str + ")");
}
}
