Algorithm

Reverse

#include <algorithm> // for std::reverse
#include <string>

int main() {
    std::string str = "Hello";
    std::reverse(str.begin(), str.end());
    std::cout << str << std::endl;  // 输出 "olleH"

Char Digit to Int

#include <string>
int main() {
    std::string digit_char = "42";
    int num_1 = std::stoi(digit_char);
    long num_2 = std::stol(digit_char);
    long long num_3 = std::stoll(digit_char);

    // single char
 char digitChar = '4';
 int digit = digitChar - '0';
    return 0;
}