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:
这道题由C实现的话相对复杂，因此学习了vector的简单实用，采用vector来解决此问题std::vector 

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;
    }
};

可用于调试版本代码如下： 

#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
 通过gdb跟踪可以对该递归方法有个清晰的认识，实现方法非常巧妙。