Valid_Number.cpp

class Solution {
public:
    bool isNumber(string s) {
        if (s.empty())
            return false;
        
        int n = (int)s.length();
        int i = 0;
        while(i < n and isspace(s[i])) {
            ++i;
        }
             
        if (s[i] == '+' || s[i] == '-')
             i++;
             
         bool eAppear = false;
         bool dotAppear = false;
         bool firstPart = false;
         bool secondPart = false;
         bool spaceAppear = false;
         
         while(i < n) {
             if (s[i] == '.') {
                 if (dotAppear || eAppear || spaceAppear)
                     return false;
                 dotAppear = true;
             } else if (s[i] == 'e' || s[i] == 'E') {
                 if (eAppear || !firstPart || spaceAppear)
                     return false;
                 eAppear = true;
             } else if (isdigit(s[i])) {
                 if (spaceAppear)
                     return false;
                     
                 if (!eAppear)
                     firstPart = true;
                 else
                     secondPart = true;
             } else if (s[i] == '+' || s[i] == '-') {
                 if (spaceAppear)
                     return false;
                     
                 if (!eAppear || !(s[i - 1] == 'e' || s[i - 1] == 'E'))
                     return false;
             } else if (isspace(s[i]))
                 spaceAppear = true;
             else
                 return false;
             i++;            
         }
         
         if (!firstPart)
             return false;
         if (eAppear && !secondPart)
             return false;
         return true;
    }
};