2 SUM I II III

3 SUM

3 SUM CLOEST

4 SUM

K SUM I II

This kind of question looks this way,

1, need find all possible solutions, we need backtracking method to find them.

2, only need to check if it is available or not, check.

3, only need for a optimist value, Dynamic programming would be very helpful here.

Usually we need dp for optimist solution, backtracking for all solutions, and another idea for avaliable

or statics solution.

Is &(&i) valid in C?

No, I would not say so. we know the expression try to get an address of

an address of a variable, however, only after compiler time construction, we

could get a value for its address, for this exact time the compiler could not

return recognize the address of a new construct value in Compiler,

So it would print error.