I'm reading the "Introduction to Algorithm", and in chapter 8.1, which is proofing the lower bound of comparison-based sorting algorithm, it says:
Consider a decision tree of height $h$ with $l$ reachable leaves corresponding to a comparison sort on $n$ elements. Because each of the $n!$ permutations of the input appears as some leaf, we have $n!\leq l$.
And my question is that Why the number of permutations ($n!$) is less than or equal to the number of reachable leaves ($l$)?
P.S. I'm learning algorithm out of interest, and don't have much knowledge about trees