Permutation and combination is a common thinking method in mathematics, which is mainly used to calculate the number of different subsets in a set composed of several elements. In this process, permutation and combination are two important concepts. Arrangement refers to the number of different ways to sort R elements from N elements, which is expressed by P(n, r); Combination refers to the number of different ways to select r elements from n elements regardless of the order, which is expressed by C(n, r).

In practical applications, permutation and combination have a wide range of applications, such as gene arrangement in biology, coding in computer science, key generation in cryptography and so on. The permutation and combination thinking method can help us solve various problems in life, work and scientific research, and improve our learning and innovation ability.

Mastering the permutation and combination thinking method requires a certain mathematical foundation, such as the permutation and combination content in junior high school mathematics and senior high school mathematics. At the same time, we need to pay attention to the difference between permutation and combination, and learn to use them flexibly to solve practical problems. In the future work and life, we can try to use the thinking method of permutation and combination to explore better solutions or optimize efficiency.