Valid Number

 1 #include <string.h>
 2 #include <iostream>
 3 #include <ctype.h>
 4 
 5 #include <fstream>
 6 
 7 using namespace std;
 8 
 9 class Solution
10 {
11 public:
12   bool isNumber(const char *s)
13   {
14     bool has_front_num = false, has_back_num = false, has_e = false;
15     int len = strlen(s);
16     int i = 0;
17 
18     while(i < len && ‘ ‘ == s[i])
19       i++;
20 
21     if(i == len) return false;
22 
23     if(i < len && (s[i] == ‘+‘ || s[i] == ‘-‘))
24       i++;
25 
26     while(i < len && isdigit(s[i]))
27     {
28       i++;
29       has_front_num = true;
30     }
31 
32     if(i < len && s[i] == ‘.‘)
33       i++;
34     
35     while(i < len && isdigit(s[i]))
36     {
37       i++;
38       has_front_num = true;
39     }
40 
41     if(i < len && (s[i] == ‘e‘ || s[i] == ‘E‘) && has_front_num)
42     {
43       i++;
44       has_e = true;
45       if(i == len) return false;
46     }
47 
48     
49     if(i < len && (s[i] == ‘+‘ || s[i] == ‘-‘) && has_e)
50       i++;
51     
52     while(i < len && isdigit(s[i]) && has_e)
53     {
54       i++;
55       has_back_num = true;
56     }
57 
58     while(i < len && s[i] == ‘ ‘)
59       i++;
60 
61     if(i == len && has_e && has_back_num)
62       return true;
63     else if(i == len && !has_back_num && !has_e && has_front_num)
64       return true;
65 
66     return false;
67   }
68 };
69 
70 int main()
71 {
72   ifstream cin("aa");
73   Solution s;
74   char *ss;
75   while(cin >> ss)
76     cout << s.isNumber(ss) << endl;
77   return 0;
78 }