NP-hard problems on directed graphs Outbranching with a maximum number of leaves Given a directed graph G = (V , A), the Directed Maximum Leaf Spanning Tree problem asks to compute a directed spanning tree with as many leaves as possible. By designing a branching algorithm analyzed with Measure&Conquer, we show that the problem can be solved in time O * (1.9044 n ) using polynomial space. Allowing exponential space, this run time upper bound can be lowered to O * (1.8139 n ). We also provide an example showing a lower-bound for the running time of our algorithm.