统计一个字符串在另一个字符串中出现的次数，包含重叠和非重叠两种情况
子串可重叠情况
int count_substring_in_string_overlapping(const std::string &str, const std::string &sub) {
int num = 0;
for (size_t i = 0; (i = str.find(sub, i)) != std::string::npos; num++, i++);
return num;
}
子串不可重叠情况
int count_substring_in_string_non_overlapping(const std::string &str, const std::string &sub) {
int num = 0;
size_t len = sub.length();
if (len == 0)len = 1;
for (size_t i = 0; (i = str.find(sub, i)) != std::string::npos; num++, i += len);
return num;
}