生成有效括号

括号生成

var generateParenthesis = function(n) {
    if(!n) return [];
    let res = [];
    function dfs(subs,left,right,n){
        if(left === n && right === n){
            res.push(subs);
            return;
        }
        if(left < right){
            return;
        }
        left < n && dfs(subs+'(',left+1,right,n);
        right < n && dfs(subs+')',left,right+1,n);
    }
    dfs('',0,0,n);
    return res;
};