Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string. Return a list of all possible strings we could create.

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output:

```class Solution
{
void backtrack(string &s, int i, vector<string> &res)
{
if (i == s.size())
{
res.push_back(s);
return;
}
backtrack(s, i + 1, res);
if (isalpha(s[i]))
{
// toggle case
s[i] ^= (1 << 5);
backtrack(s, i + 1, res);
}
}

public:
vector<string> letterCasePermutation(string S)
{
vector<string> res;
backtrack(S, 0, res);
return res;
}
};
```

```#include <iostream>
#include <vector>

using namespace std;
void backtrack(string &s, int i, vector<string> &res)
{
if (i == s.size())
{
res.push_back(s);
return;
}
backtrack(s, i + 1, res);
if (isalpha(s[i]))
{
// toggle case
s[i] ^= (1 << 5);
backtrack(s, i + 1, res);
}
}

int main()
{
string s="abc";
vector<string> res;
backtrack(s,0,res);
for (std::vector<string>::iterator it = res.begin() ; it != res.end(); ++it)
{
std::cout << ' ' << *it;
}

std::cout << '\n';
return 0;
}
```

g++ -g leetcode.cpp
gdb ./a.out