LeetCode题解： Valid Number

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.

思路：

状态机而已。不过略微有点复杂。当然也可以用正则来做。正则表达式：

FP_REGEX = / +[-+]?[0-9]*\.?[0-9]+([eE][-+]?[0-9]+)? +/

不过LeetCode不准用，所以这里借用JSON网站上对数的定义：

题解：

class Solution {
public:
    enum State : int
    {
        TERM = 0,
        SIGN = (1 << 0),
        NUMBER = (1 << 1),
        POINT = (1 << 2),
        EXPSGN = (1 << 3),
        BLANK = (1 << 8),
        INVALID = -1
    };
    
    State currentCharState (char ch)
    {
        switch (ch)
        {
        case '+':
        case '-':
            return SIGN;
        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        case '8':
        case '9':
            return NUMBER;
        case '.':
            return POINT;
        case 'E':
        case 'e':
            return EXPSGN;
        case ' ':
            return BLANK;
        case 0:
            return TERM;
        default:
            return INVALID;
        }
    }
    
    enum States : int
    {
        S0 = SIGN | NUMBER | POINT | BLANK,
        S1 = NUMBER | POINT,
        S2 = NUMBER | POINT | EXPSGN | BLANK | TERM,
        S3A = NUMBER,
        S3 = NUMBER | EXPSGN | BLANK | TERM,
        S4 = SIGN | NUMBER,
        S5 = NUMBER,
        S6 = NUMBER | BLANK | TERM,
        S7 = BLANK | TERM,
        S8 = TERM
    };
    
    map<States, map<State, States>> STATE_MACHINE =
    {
        {S0, {{SIGN, S1}, {NUMBER, S2}, {POINT, S3A}, {BLANK, S0}}},
        {S1, {{NUMBER, S2}, {POINT, S3A}}},
        {S2, {{NUMBER, S2}, {POINT, S3}, {EXPSGN, S4}, {BLANK, S7}, {TERM, S8}}},
        {S3A, {{NUMBER, S3}}},
        {S3, {{NUMBER, S3}, {EXPSGN, S4}, {BLANK, S7}, {TERM, S8}}},
        {S4, {{SIGN, S5}, {NUMBER, S6}}},
        {S5, {{NUMBER, S6}}},
        {S6, {{NUMBER, S6}, {BLANK, S7}, {TERM, S8}}},
        {S7, {{BLANK, S7}, {TERM, S8}}},
        {S8, {{TERM, S8}}}
    };
    
    bool isNumber (const char* s)
    {
        if (s == nullptr)
            return false;
            
        States stat = S0;
        for (;;)
        {
            char ch = *s++;
            State ch_stat = currentCharState (ch);
            auto next_stat_iter = STATE_MACHINE[stat].find (ch_stat);
            if (ch_stat == INVALID || next_stat_iter == STATE_MACHINE[stat].end())
                break;
            stat = next_stat_iter->second;
        }
        return (stat == S8);
    }
};