LeetCode 246. Strobogrammatic Number

WIki on what is Strobogrammatic number: https://en.wikipedia.org/wiki/Strobogrammatic_number

#include <iostream>
#include <string>
using namespace std;

bool valid(char a, char b) {
  switch(a) {
    case '0':
      return b == '0';
    case '1':
      return b == '1';
    case '8':
      return b == '8';
    case '6':
      return b == '9';
    case '9':
      return b == '6';
  }
}


bool isStrobogrammatic(string num) {
  if(num.size() == 0) return true;
  int i = 0;
  int j = num.size() -1;
  while(i <= j) {
    char a = num[i];
    char b = num[j];
    if(!valid(a, b)) return false;
    i++;
    j--;
  }
  return true;
}

int main(void) {
  cout << isStrobogrammatic("619") << endl;
}