Given a string S, return the "reversed" string where all characters that are not a letter stay in the same place, and all letters reverse their positions.

 

 

Example 1:

Input: "ab-cd"Output: "dc-ba"

Example 2:

Input: "a-bC-dEf-ghIj"Output: "j-Ih-gfE-dCba"

Example 3:

Input: "Test1ng-Leet=code-Q!"Output: "Qedo1ct-eeLg=ntse-T!"

 

Note:

1. S.length <= 100
2. 33 <= S[i].ASCIIcode <= 122 
3. S doesn't contain \ or "

思路：

两个标记，分别从前和从后往中间方向遍历，遇到字母交换，否则游标继续往下走。

bool isLetter(char ch){    return ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'));}string reverseOnlyLetters(string S){    string ret = S;    int n = S.length();    int start = 0;    int end = n - 1;    while(start <= n-1 && end >= 0)    {        if(!isLetter(S[end])){end--;continue;}        if(!isLetter(ret[start])){start++;continue;}        ret[start++] = S[end--];    }     return ret;       }