Prove that every tournament contains at least one Hamiltonian path. A tournament is a directed graph with exactly one edge between every pair of vertices. (So for each pair  (u,v)  of vertices, either the edge from  u to  v  or from  v  to  u  exists, but not both.) You can think of the nodes as players in a tournament, where each player has to play against every other player. The edge points from the winner to the loser of a game. 
A Hamiltonian path (not cycle) is a sequence of consecutive directed edges that visits every vertex exactly once.
How can i prove that every tournament contains at least one Hamiltonian path?
thanks your help!  

This can be proven using strong induction. Let $n$ be the number of vertices. When $n \le 2$, a hamiltonian path clearly exists. Now, for any given $n > 2$, pick any arbitrary vertex $v$. Partition all other vertices other than $v$ into the sets $V_{out}$ and $V_{in}$ -- $V_{out}$ contains all other vertices $u$ such that the edge $(v, u)$ exists, while $V_{in}$ contains all other vertices $u$ such that the edge $(u, v)$ exists.
Clearly $|V_{out}| < n$ and $|V_{in}| < n$, so by the inductive hypothesis, there is a hamiltonian path in both sets. Let $H_{out}$ be any hamiltonian path in $V_{out}$ and $H_{in}$ be any hamiltonian path in $V_{in}$. You can then form a hamiltonian path for all vertices by concatenating $H_{in}$, $v$, and $H_{out}$.
