思路：模拟正则表达式
    状态机如下：
    TIM图片20200326123834.jpg

    1.     bool isNumeric(char* string)
    2.     {
    3.         int i = 0;
    4.         if(string[i]=='+' || string[i]=='-' || IsNum(string[i])){
    5.             while(string[++i]!='\0' && IsNum(string[i]));
    6.             if(string[i]=='.'){
    7.                 if(IsNum(string[++i])){
    8.                     while(string[++i]!='\0' && IsNum(string[i]));
    9.                     if(string[i]=='e'||string[i]=='E'){
    10.                         i++;
    11.                         if(string[i]=='+' || string[i]=='-' || IsNum(string[i])){
    12.                             while(string[++i]!='\0' && IsNum(string[i]));
    13.                             if(string[i]=='\0') return true;
    14.                             else return false;
    15.                         }else return false;
    16.                     }else if(string[i]=='\0') return true;
    17.                     else return false;
    18.                 }else if(string[++i]=='\0') return true;
    19.                 else return false;
    20.             }else if(string[i]=='e'||string[i]=='E'){
    21.                 i++;
    22.                 if(string[i]=='+' || string[i]=='-' || IsNum(string[i])){
    23.                     while(string[++i]!='\0' && IsNum(string[i]));
    24.                     if(string[i]=='\0') return true;
    25.                     else return false;
    26.                 }else return false;
    27.             }else if(string[i]=='\0') return true;
    28.             else return false;           
    29.         }else return false;
    30.     }
    32.     bool IsNum(char ch)
    33.     {
    34.         if(ch<'0'||ch>'9') return false;
    35.         else return true;
    36.     }