方法：双指针

class Solution {
public:
    bool isletter(char ch)
    {
        if(ch>='a'&&ch<='z')
            return true;
        if(ch>='A'&&ch<='Z')
            return true;
        return false;
    }
    string reverseOnlyLetters(string s) {
        int len=s.size();
        int left=0,right=len-1;
        string s1;
        while(left<len)
        {
            if(isletter(s[left]))
            {
                while(!isletter(s[right]))
                    --right;
                s1+=s[right];
                ++left;
                --right;
            }
            else
            {
                s1+=s[left];
                ++left;
            }
        }
        return s1;
    }
};

2. 

class Solution {
public:
    bool isletter(char ch)
    {
        if(ch>='a'&&ch<='z')
            return true;
        if(ch>='A'&&ch<='Z')
            return true;
        return false;
    }
    string reverseOnlyLetters(string s) {
        if(s.empty())
            return s;
        size_t left=0,right=s.size()-1;
        while(left<right)
        {
            while(left<right&&!isletter(s[left]))
                ++left;
            while(left<right&&!isletter(s[right]))
                --right;
            swap(s[left],s[right]);
            ++left;
            --right;
        }
        return s;
    }
};