II. (50 points) There are 26 stamp collectors from different countries who want to exchange the latest commemorative stamps issued by their respective countries through correspondence. To ensure that each of the 26 individuals ends up with a complete set of the latest commemorative stamps from all 26 countries, what is the minimum number of letters they need to send?
II. Solution: Using the principle of counting twice and the principle of abstraction. Without loss of generality, let one of the 26 stamp collectors be the leader. On one hand, for the leader to receive the latest commemorative stamps from the other 25 countries, these must be sent by the other 25 collectors (regardless of whether they send directly to the leader), which requires at least 25 letters; similarly, the other 25 collectors each need to receive a set of the latest commemorative stamps from the leader, which must be sent by the leader (regardless of whether he sends directly to the collectors), requiring at least 25 letters, totaling at least 50 letters. On the other hand, the other 25 collectors each send 25 sets of the latest commemorative stamps from their own country to the leader, totaling 25 letters. After the leader receives these 25 letters, he then sends one letter to each of the 25 collectors, each containing 25 sets of stamps (excluding the set from the collector's own country) or 26 sets of stamps (including the set from the collector's own country), totaling 25 letters, making a total of 50 letters. This means that 50 letters can ensure that each of the 26 collectors has a set of the latest commemorative stamps from 26 countries. Therefore, at least 50 letters need to be sent.