Valid Number

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.

Solution: This finite-state machine solution. Learn from fuwutu & snakeDling.

 1 class Solution {
 2 public:
 3     bool isNumber(const char *s) {
 4         enum InputType {INVALID, SPACE, SIGN, DIGIT, DOT, EXPONENT};
 5         int transitionTable[][6] = 
 6         { /* 0   1   2   3   4   5  */
 7              0,  1,  2,  3,  4,  0, // 0: INVALID
 8              0,  1,  2,  3,  4,  0, // 1: SPACE
 9              0,  0,  0,  3,  4,  0, // 2: SIGN
10              0,  6,  0,  3,  7,  5, // 3: DIGIT
11              0,  0,  0,  7,  0,  0, // 4: DOT
12              0,  0,  2,  8,  0,  0, // 5: EXPONENT
13              0,  6,  0,  0,  0,  0, // 6: END WITH SPACE
14              0,  6,  0,  7,  0,  5, // 7: DOT AND DIGIT
15              0,  6,  0,  8,  0,  0, // 8: END WITH SPACE OR DIGIT
16         };
17         
18         InputType last = INVALID;
19         while (*s != '')
20         {
21             InputType state = INVALID;
22             if (*s == ' ')
23                 state = SPACE;
24             else if (isdigit(*s))
25                 state = DIGIT;
26             else if (*s == '+' || *s == '-')
27                 state = SIGN;
28             else if (*s == 'e')
29                 state = EXPONENT;
30             else if (*s == '.')
31                 state = DOT;
32             last = (InputType) transitionTable[last][state];
33             if (last == INVALID) return false;
34             s++;
35         }
36         bool validFinal[] = {0, 0, 0, 1, 0, 0, 1, 1, 1};
37         return validFinal[last];
38     }
39 };