function next(p){
    var i = p.length - 1;
    while(i + 1 && p[i] < p[--i]);
    if(i + 1){
        var j = p.length;
        while(p[--j] < p[i]);
        var s = p[j];
        p[j] = p[i];
        p[i] = s;
        j = 1;
        while(i + 2*j < p.length){
            s = p[i + j];
            p[i + j] = p[p.length - j];
            p[p.length - j++] = s;
        }
        return p;
    }else{
        return null;
    }
}