已知向量 $\stackrel{\rightharpoonup }{a}=(0,-1,1),\stackrel{\rightharpoonup }{b}=(4,1,0)$， $\left|\lambda \stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right|=\sqrt{29}$且 $\lambda >0$，则 $\lambda =$．