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”]

C++程式碼:

class Solution
{
public:
  vector<string> letterCasePermutation(string S) {
    vector<string> ans;
    DFS(ans , S , 0);
    return ans;
  }
private:
  void DFS(vector <string> &ans, string &
 
S,int i){
    if(i == S.length()) {
      ans.push_back(S);
      return ;
    }
    DFS(ans,S,i+1);
    if(isalpha(S[i])){
      S[i] ^= (1<<5);  //a->A,A->a
      DFS(ans,S,i+1);
      S[i] ^=(1<<5); //這裡可要可不要,不需要回溯, 不恢復狀態也沒有關係
    } else{
      return;
    }
  }
};

Complexity Analysis:

Time complexity : O( $n*2^{l}$). ( n: 字串長度, l: 字元數量. 最好的情況;全都是數字;最壞的情況:全都是字母.)
Space complexity : O( $n$) + O( $n*2^{l}$)stack.

思路:

• DFS思想
• 如下圖: