1. In terms of time, the United States needs 4 days and the United States needs 3 days.

2. On the topic, the United States gives many choices, mostly focusing on practical problems. Especially for E and F questions in the United States, the requirements for solving the algorithm are not very high, and the focus is on the construction of the model. Every school in the national competition has a limit on the number of winners, such as several national prizes and several provincial prizes; No, everything is for fun.

3. In terms of requirements, China Tournament generally requires specific results, that is, programmers should complete the program design and give correct answers. However, the United States can't give an answer, that is to say, it can't give an exact answer to some questions.