struct Node {
    ll l , r;
    mutable ll v;
    Node(ll l, ll r = 0, ll v = 0) : l(l), r(r), v(v) {}
    bool operator<(const Node &a) const {
        return l < a.l;
    }
};

这段代码里的 Node(ll l, ll r = 0, ll v = 0) : l(l), r(r), v(v) {}是什么意思