Idea: When searching, build an index (Hash search) or sort (binary search). The cache of this question can be indexed during the search process, so a loop can be used to find the solution (always use unused
elements to find used elements to ensure that every pair is retrieved). Indexing/ordering is the first step to search questions.
2. Add Two Numbers
Idea: Digital carry problem, the effective value of this bit is the value %10, and the carry value is the value /10. You can use a variable to record the carry value