[C/C++] 简单实现trim函数：删除字符串头尾空格

记录一下

string trim(const string &s) {
    int start = 0, end = s.size() - 1;
    while (start < s.size() && s[start] == ' ') {
        start++;
    }
    while (end >= 0 && s[end] == ' ') {
        end--;
    }
    return start <= end ? s.substr(start, end - start + 1) : "";
}