巧用nth_element求容器前n%大小的那个数

 #include <algorithm>
 #include <vector>
 #include <iostream>
 #include <string>
 #include <numeric>
 std::string::size_type stringLengthSum(std::string::size_type sumSoFar, const std::string& s)
 {
     return sumSoFar += s.size();
 }
 int main()
 {
     std::vector<int> v{,,,,,,,,,,,};
     auto begin = v.begin();
     auto end = v.end();
     decltype(begin) goodPosition;
     goodPosition = begin + 0.3 * v.size();
     nth_element(begin, goodPosition, end);
     std::cout << *goodPosition << std::endl;
     return ;
 }