Rollback DSU

struct ROLLBACK_DSU {
    vector<int>par, sz;
    vector<pair<int,int>>op;

    ROLLBACK_DSU(int n) : par(n+1), sz(n+1, 1) {
        iota(par.begin(), par.end(), 0);
    }
    int find(int v) {
        return (v==par[v] ? v : find(par[v]));
    }
    void unite(int u, int v) {
        u = find(u), v = find(v);
        if(sz[u] < sz[v]) swap(u, v);

        op.emplace_back(u, v);
        par[v] = u, sz[u] += sz[v];
    }
    void UNDO(int t) {
        while(op.size()>t) {
            auto [u, v] = op.back();
            op.pop_back();
            par[v] = v, sz[u] -= sz[v];
        }
    }
};