valid_number.cpp

class Solution {
public:
    bool isNumber(string s) 
{
    int i = 0;
    
    // skip the whilespaces
    for(; s[i] == ' '; i++) {}
    
    // check the significand
    if(s[i] == '+' || s[i] == '-') i++; // skip the sign if exist
    
    int n_nm, n_pt;
    for(n_nm=0, n_pt=0; (s[i]<='9' && s[i]>='0') || s[i]=='.'; i++)
        s[i] == '.' ? n_pt++:n_nm++;       
    if(n_pt>1 || n_nm<1) // no more than one point, at least one digit
        return false;
    
    // check the exponent if exist
    if(s[i] == 'e') {
        i++;
        if(s[i] == '+' || s[i] == '-') i++; // skip the sign
        
        int n_nm = 0;
        for(; s[i]>='0' && s[i]<='9'; i++, n_nm++) {}
        if(n_nm<1)
            return false;
    }
    
    // skip the trailing whitespaces
    for(; s[i] == ' '; i++) {}
    
    return s[i]==0;  // must reach the ending 0 of the string
}
};