思路：先换成整数，再乘，再除

function multiply(a, b) {
    let s1=a.toString(), s2=b.toString(), m=0;
    if(s1.includes('.')){
        m+=s1.split('.')[1].length
    }
    if(s2.includes('.')){
        m+=s2.split('.')[1].length
    }
    return s1.replace('.','')*s2.replace('.','')/Math.pow(10,m)
}