[leetcode-917-Reverse Only Letters]

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;       
}