22. Generate Parentheses

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:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]
题意：求n双括号的所有有效组合
代码如下：

/**
 * @param {number} n
 * @return {string[]}
 
回溯
 */
var generateParenthesis = function(n) {
    var res=[];
    back(res,"",0,0,n);
    
    return res;
};
//list :结果数组
//str :收集结果的某一项
//left:左括号
//right:右括号
//n:括号对数
function back(list,str,left,right,n){
    if(str.length==2*n){
        list.push(str);
        return ;
    }
    if(left<n) back(list,str+"(",left+1,right,n);
    if(left>right) back(list,str+")",left,right+1,n);
}