22. Generate Parentheses.md

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:

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

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res=new ArrayList<>();
        String curr="";
        backtrack(res,curr,0,0,n);
        return res;
    }
    public void backtrack(List<String> res,String curr,int open,int close,int max){
        if(curr.length()==max*2){
            res.add(curr);
            return;
        }
        if(open<max) backtrack(res,curr+"(",open+1,close,max);
        if(close<open) backtrack(res,curr+")",open,close+1,max);
    }
}