Problem description:

Validate if a given string is numeric.

Some examples:
“0” => true
“ 0.1 “ => true
“abc” => false
“1 a” => false
“2e10” => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

Update (2015-02-10):
The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button to reset your code definition.

Solution:

This is a problem to let you discuss with interviewer and implement the detail.

  1. skip the leading white spaces
  2. check the positive or negative sign is valid. if there’re two ‘+’ or ‘-‘, it should be invalid.
  3. To do so, simply skip the leading sign and count the number of digits and the number of points. A valid significant has no more than one point and at least one digit.
  4. check if the exponent part is valid. We do this if the significant is followed by ‘e’. Simply skip the leading sign and count the number of digits. A valid exponent contain at least one digit.
  5. skip the trailing white spaces. We must reach the ending ‘\0’ if the string is a valid number.
  • White spaces: may have three cases
  1. leading white spaces, skip it
  2. trailing white space, skip it
  3. white spaces in the middle of string, invalid number
  • Decimal point
  1. can only appear once, but can show in any position. “.3”, “1.e2”, “1.”
  2. can not showed after e. “1e.1”

example that can use

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
string s1 = "0"; // True
string s2 = " 0.1 "; // True
string s3 = "abc"; // False, not digit
string s4 = "1 a"; // False, because white space in the middle
string s5 = "2e10"; // True

string s6 = "-e10"; // False
string s7 = " 2e-9 "; // True
string s8 = "+e1"; // False
string s9 = "1+e"; // False
string s10 = " "; // False

string s11 = "e9"; // False
string s12 = "4e+"; // False
string s13 = " -."; // False
string s14 = "+.8"; // True
string s15 = " 005047e+6"; // True

string s16 = ".e1"; // False
string s17 = "3.e"; // False
string s18 = "3.e1"; // True
string s19 = "+1.e+5"; // True
string s20 = " -54.53061"; // True

string s21 = ". 1"; // False

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
class Solution {
public:
    bool isNumber(string s) {
        //"1 1"is not valid, "0x11" is not valid, "." is not valid, "2e3.1" is not valid, "2e" is not valid,"1a" is not valid,"++1" is not valid
        
        
        int i = 0;
        // skip the whilespaces
        while(s[i] == ' ') i++;

        // check the significand
        // skip the sign if exist
        if(s[i] == '+' || s[i] == '-') i++; 

        int n_nm= 0; //how many digits
        int n_pt= 0; //how many decimal points
        while(isdigit(s[i]) || s[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++;
            // skip the sign, " 005047e+6", it should also be true
            if(s[i] == '+' || s[i] == '-') i++; 

            int n_nm = 0;
            while(isdigit(s[i])){
                i++;
                n_nm++;
            }
            if(n_nm<1)
                return false;
        }

        // skip the trailing whitespaces
        while(s[i] == ' ')
            i++;
        
        // must reach the ending 0 of the string
        return s[i]== '\0'; 
    }
};

time complexity: $O(n)$

reference:
https://goo.gl/qsSdzM
https://goo.gl/s91yxH