// 给定n个括号随机组合
const res = []
function fn(left, right, str) {
if (left ===0 && right === 0) return res.push(str)
if (left > 0) {
fn(left - 1, right, str + '(')
}
if (left < right) {
fn(left, right - 1, str + ')')
}
}
fn(4,4, '')
