65. Valid Number

Validate if a given string can be interpreted as a decimal number.

Some examples:
"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true
" -90e3   " => true
" 1e" => false
"e3" => false
" 6e-1" => true
" 99e2.5 " => false
"53.5e93" => true
" --6 " => false
"-+3" => false
"95a54e53" => false

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one. However, here is a list of characters that can be in a valid decimal number:

• Numbers 0-9
• Exponent - "e"
• Positive/negative sign - "+"/"-"
• Decimal point - "."

Of course, the context of these characters also matters in the input.

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.

 AC code:

class Solution {
public:
    bool isNumber(string s) {
        int i = 0;
        int n_nm = 0, n_pt = 0;
        
        for ( ; s[i] == ' '; i++) {}
        if (s[i] == '-' || s[i] == '+') i++;
        
        for ( ; (s[i] <= '9' && s[i] >= '0') || s[i] == '.'; ++i) {
            if (s[i] == '.')
                n_pt++;
            else 
                n_nm++;
        }
        if (n_nm < 1 || n_pt > 1) return false;
        
        if (s[i] == 'e') {
            i++;
            if (s[i] == '-' || s[i] == '+') i++;
            
            n_nm = 0;

            for ( ; s[i] <= '9' && s[i] >= '0'; ++i) {
                    n_nm++;
            }
            if (n_nm < 1) return false;
        }
            
        for ( ; s[i] == ' '; i++) {}
        return s[i] == 0;
    }
};

Runtime: 16 ms, faster than 76.66% of C++ online submissions for Valid Number.