reduce some compile warning

[jacquesqiao]

fix #3644

[JiayiFeng]

Don't use size_t in for loop: https://google.github.io/styleguide/cppguide.html#Integer_Types

You can write like this:

for (int i = 0; i < int(finalPaths_.size()); ++i) {
  for (int j = 0; j < int(finalPaths_[i].size()); ++j) {
    // ...
  }
}

[jacquesqiao]

do not quit understand why it's not good to use size_t in a for loop. Instead it writes:
When appropriate, you are welcome to use standard types like size_t and ptrdiff_t.

and here has some other explanation.
https://stackoverflow.com/questions/1951519/when-should-i-use-stdsize-t

[JiayiFeng]

Maybe we can follow this principle:
If you want to traverse a vector or an array with for loop, size_t is acceptable, otherwise use int instead.

[jacquesqiao]

ok

[reyoung]

@Canpio I think we should not obey google c++ style for this situation.

Because all STL containers are rewritten in google. They make containers' size_type as int. We are using std's containers like unordered_map, vector, which size_type is size_t.

So maybe just use size_t is fine. Using int only has an advantage when reversely access a container,i.e., when uses -- operator.