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: SIGN10              0,  6,  0,  3,  7,  5, // 3: DIGIT11              0,  0,  0,  7,  0,  0, // 4: DOT12              0,  0,  2,  8,  0,  0, // 5: EXPONENT13              0,  6,  0,  0,  0,  0, // 6: END WITH SPACE14              0,  6,  0,  7,  0,  5, // 7: DOT AND DIGIT15              0,  6,  0,  8,  0,  0, // 8: END WITH SPACE OR DIGIT16         };17         18         InputType last = INVALID;19         while (*s != '\0')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 };