22. Generate parentheses

/* *  Numbers  n  Represents the logarithm of the generated bracket , Please design a function , Used to be able to generate all possible and   Effective   Bracket combination . *  Valid parenthesis ： *  Input ：n = 3 *  Output ：["((()))","(()())","(())()","()(())","()()()"] *  Input ：n = 1 *  Output ：["()"] * * *  From the process of simulation ： *  First, the string is empty , You can add (、 You can also add ) *  When the iteration addition ends , namely str The length of is 2n, Or invalid ; *  When is it invalid ？  That is, the number of left parentheses is less than that of right parentheses , In other cases, you can continue to iterate  *  When continuing the iteration , Sometimes you can add (, When can I add ) * ( The number of and n relevant ,) The number of is related to the number of left parentheses , Should be satisfied with :r<=l<=n,=>l<n,r<l Is the foundation of iteration  * *  difficulty ： Abstract the relationship between the left and right brackets , And the basis of iteration  * */
var generateParenthesis = function(n) {
    
    var result = [],l=0,r=0,str='';
    back_tracking(n,result,l,r,str);
    return result;
};
var back_tracking=function(n,result,l,r,str){
    
    if(l<r){
    
        return;
    }
    if(str.length===2*n){
    
        result.push(str);
        return;
    }
    if(l<n){
    
        back_tracking(n,result,l+1,r,str+'(');
    }
    if(l>=r){
    
        back_tracking(n,result,l,r+1,str+')');
    }

}
console.log(generateParenthesis(1))