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.

将S的每个字母进行大写小小写转换，求出所有的可能

C++（9ms）：  BFS

class Solution {
public:
    vector<string> letterCasePermutation(string S) {
        queue<string> que ;
        que.push(S) ;
        for(int i = 0 ; i < S.size() ; i++){
            if (isdigit(S[i]))
                continue ;
            int len = que.size() ;
            for (int j = 0 ; j < len ; j++){
                string cur = que.front() ;
                que.pop() ;
                
                cur[i] = toupper(cur[i]);
                que.push(cur) ;
                
                cur[i] = tolower(cur[i]) ;
                que.push(cur) ;
            }
        }
        vector<string> res ;
        int len = que.size() ;
        for(int i = 0 ; i < len ; i++){
            res.push_back(que.front()) ;
            que.pop() ;
        }
        return res ;
    }
};