求最长相同特征（不相连）

//求最长相同子特征
function LSC (s1, s2) {
    function getNum(n1,n2) {
        if(n1<0||n2<0){
            return {
                s1Arr:[],
                s2Arr:[],
                num:0
            }
        }
        const is=s1[n1]===s2[n2]?1:0;
        if(is){
            const l1=getNum(n1-1,n2-1);
            l1.s1Arr.push(n1)
            l1.s2Arr.push(n2)
            l1.num=l1.num+1;
            return l1
        }else{
            const l1=getNum(n1-1,n2-1);
            const l2=getNum(n1-1,n2);
            const l3=getNum(n1,n2-1);
            let lMax=l1;
            if(l2.num>l1.num&&l2.num>l3.num){
                lMax=l2;
            }else if(l3.num>l1.num&&l3.num>l2.num){
                lMax=l3;
            }
            return lMax;
        }
    }
    const obj=getNum(s1.length-1,s2.length-1)
    let ctr='';
    obj.s1Arr.forEach(function (n) {
        ctr+=s1.substr(n,1)
    })
    return ctr
}

console.log(LSC('121213121,1421','1,21'))