JS几乎不用考虑时间复杂度太高。

const readline = require('readline')

const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});
// 监听键入回车事件
var a=new Array(),k=0;
rl.on('line', (str) => {
    // str即为输入的内容
    if (str === 'close') {
        // 关闭逐行读取流 会触发关闭事件
        rl.close()
    }
    a[k++]=str;//保存读入数据
});

rl.on('close', () => { //输入完毕
    //输入完后处理。
    //输入完后变量为a。

    var ul=false;
    for(var i=0;i<a.length;i++){
        var em=false,url=false,bold=false;
        var nowline=a[i];
        if(nowline===""){
            if(ul===true){
                console.log("</ul>");
                ul=false;
            }
            continue;
        } 
        var cnt=0;
        var j=0;
        if(nowline[j]=='*'){
            if(nowline[j+1]==' '){
                if(ul===false){
                    console.log("<ul>");
                }
                ul=true;
            }
        }else{
            if(ul===true){
                console.log("</ul>");
            }
            ul=false;
        }
        j=0;
        while(j<nowline.length){
            if(nowline[j]=='_'){

                var be=nowline.substr(0,j);
                var af=nowline.substr(j+1,nowline.length-j-1);
                nowline=be+"<"+(em?"/":"")+"em>"+af;
                em=!em;
            }

            if(nowline[j]=='['){
                var afk=-1;
                var m1=-1;

                for(var k=j+1;k<nowline.length;k++){
                    if(nowline[k]==']' && afk==-1) afk=k;
                    else if(nowline[k]==')' && m1==-1){
                        m1=k;
                        break;
                    }
                }

                try{
                    if(m1==-1 || afk==-1){

                    }else{
                        //console.log(afk,":",m1-afk-2-1,":",m1,":");
                        var be=nowline.substr(0,j);
                        var af=nowline.substr(m1+1,nowline.length-m1-1);
                        var url=nowline.substr(afk+2,m1-afk-2);
                        var value=nowline.substr(j+1,afk-j-1);
                        nowline=be+"<a href=\""+url+"\">"+value+"</a>"+af;
                        j++;
                    }

                }catch(e){

                }

                //console.log(be,":",af,":",url,":",value);
            }
            j++;
        }
        for(var j=0;j<=6;j++){
            if(nowline[j]!='#'){
                if(nowline[j]!=' ') cnt=0;
                break;
            }else{
                cnt++;
            }
        }
        if(cnt!==0){
            nowline="<h"+cnt+">"+nowline.substr(cnt+1,nowline.length-cnt-1)+"</h"+cnt+">"
        }else{

            if(!ul){
                if(i==0) nowline="<p>"+nowline;
                else if(a[i-1]==""){
                    nowline="<p>"+nowline;
                }
                if(i==a.length-1) nowline=nowline+"</p>";
                else if(a[i+1]==""){
                    nowline=nowline+"</p>";
                }
            }
            else
                nowline="<li>"+nowline.substr(2,nowline.length-2)+"</li>";
        }
        var out="";
        for(var j=0;j<nowline.length;j++){
            if(nowline[j]=='>'){
                for(var k=j+1;k<nowline.length;k++){
                    if(nowline[k]!=' '){
                        nowline=nowline.substr(0,j+1)+nowline.substr(k,nowline.length-k);
                        break;
                    }
                }
                break;
            }
        }
        console.log(nowline);
        if(ul===true && i==a.length-1){
            console.log("</ul>");
        }
    }
});