[leetcode-784-Letter Case Permutation]

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: ["12345"]

Note:

• S will be a string with length at most 12.
• S will consist only of letters or digits.

思路：

深度优先遍历。

void dfs (vector<string>& ret,string S,int i)
{
    if(i >= S.length())
    {
        ret.push_back(S);
        return;
    }
    
    if(isalpha(S[i]))
    {
        S[i] = tolower(S[i]);
        dfs(ret,S,i+1);
        
        S[i] = toupper(S[i]);
        dfs(ret,S,i+1);
    }
    else dfs(ret,S,i+1);
    
}
vector<string> letterCasePermutation(string S)
{
    vector<string> ret;
    dfs(ret,S,0);    
    return ret;
}

参考：

https://leetcode.com/problems/letter-case-permutation/discuss/117180/clean-C++-solution.