全网唯一递归法：

vector<vector<int>> generate(int numRows) {
    vector<int> v;
    vector<vector<int>>vn;

    if (numRows == 1)
    {
        v.push_back(1);
        vn.push_back(v);
        v.clear();
        return vn;//递归记得return
    }
    if (numRows == 2)
    {
        v.push_back(1);
        vn.push_back(v);
        v.clear();
        v.push_back(1);
        v.push_back(1);
        vn.push_back(v);
        v.clear();
        return vn;//递归记得return
    }
    else
    {
        vn = generate(numRows - 1);//递归精髓
        v = vn[numRows - 2];
        vector<int> v2;
        v2.resize(numRows, 1);
        for (int i = 1; i < numRows - 1; i++)
        {
            v2[i] = v[i-1] + v[i];
        }
        vn.push_back(v2);
        v.clear();
    }
    return vn;
}