1、简单做法，将三种情况分别列出来。

class Solution {
public:
    bool detectCapitalUse(string word) {
       int i=1;
       if(word[i]=='\0') return true;
       else if(word[0]<=90&&word[0]>=65&&word[i]<=122&&word[i]>=97)
        {
            while(word[i]!='\0')
            {
                 if(word[i]<=90&&word[i]>=65) return false;
                 ++i;
            }
            return true;
        }
        else if(word[0]<=90&&word[0]>=65)
        {
            i=1;
            while(word[i]!='\0')
            {
                 if(word[i]<=122&&word[i]>=97) return false;
                 ++i;
            }
            return true;
        }
        else
        {
            i=0;
            while(word[i]!='\0')
            {
                 if(word[i]<=90&&word[i]>=65) return false;
                 ++i;
            }
            return true;
        }
    }
};