leetcode 22 Generate Parentheses

题目

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:

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

解释

这是n = 2的状况.就是从 ( 开始遍历,假如 ( 的数目大于 ) 的数目 那就返回,这样是不符合条件的,(ps:这里的数目是指剩余的数目,若是左括号剩余的多,那说明已经匹配好的字符串中多了右括号,这种状况没法经过继续添加括号来知足条件,因此要返回,就像图中最右边那个节点同样)而后其余的就是照常遍历就ok.知足left == 0 && right == 0 push到res里面.

代码

/**
 * @param {number} n
 * @return {string[]}
 */
var generateParenthesis = function(n) {
    var res = [];
    helper(res, "", n, n);
    return res;
};

var helper = (res, s, left, right) => {
    if (left > right) {
        return;
    }
    
    if (left == 0 && right == 0) {
        res.push(s);
        return;
    }
    if (left > 0) {
        helper(res, s+"(", left-1, right);
    }
    if (right > 0) {
        helper(res, s+")", left, right-1);
    }
};