偶数子序列

let allarr;
let alltemp,restemp=[];
let nn,kk;
let res;
function begin(n){
    alltemp=new Array(n).fill(0)
}
// function clear(){
//     alltemp==new Array(nn).fill(0)
//     restemp=[]
// }

function fn(x,s){
    // console.log("restemp:"+restemp);
    // console.log("x:"+x);
    if(kk==x){
        // console.log("kk x restemp:"+restemp);
        // console.log("kk:"+kk);
        // console.log("x:"+x);
        if(s%2==0)res++;
        return ;
    }
    for (let i = 0; i < nn; i++) {
        if(!alltemp[i]){
            restemp.push(allarr[i])
            alltemp[i]=1
            s+=allarr[i]
            // let y=x+1
            fn(x+1,s)
            alltemp[i]=0
            restemp.pop()
        }
    }
}

function main(n,k,arr){
    res=0
    allarr=arr;
    nn=n;
    kk=k;
    begin(n);
    fn(0,0)
    return res/jiecheng(k);
}

function jiecheng(x){
    if(x==1)return 1;
    return x*jiecheng(x-1)
}
console.log(main(5,2,[1,2,3,4,5]));
console.log(main(3,1,[1,2,3]));
console.log(main(4,3,[1,4,2,2]));