c++ - fast way for string comparison -

i have simple question makes me confused.

i have 2 strings, , want count how many different characters between two. strings sorted, equal length. not split strings.

for example

input:  abc, bcd output: 2, because , d different characters  input:  abce, bccd output: 4, because a, c, d , e different. 

i know can in o(n^2), how can solve in o(n) these sorted strings?

only need number of different characters, no need indicate number.

i thinking needed complicated algorithm, smith-waterman example. restrictions on input makes easy implement in o(m + n), m length of first string, , n length of second string.

we can use builtin algorithm calculate number of characters in common, , can use information produce number looking for:

#include <algorithm> #include <iostream> #include <string>  int main() {     std::string m = "abce";     std::string n = "bccd";     std::string result;      std::set_intersection(             m.begin(), m.end(),             n.begin(), n.end(),             std::back_inserter(result));      std::cout << m.size() + n.size() - 2 * result.size() << "\n"; } 

in particular case, outputs 4, wanted.