「Leetcode」字符串相关问题选编

剑指 Offer 20. 表示数值的字符串

这题介绍一个使用有限自动机的做法，实际上细心的考虑corner case的效率比这个方法更好，但是这个方法比较具有代表性。
画出如下的示意图：

然后照着这个示意图做就可以了。
注意，这么几种样例都是合法的：

-1.e-12
+.1
+1.
-.52E+3
114.514e123

接下来见代码：

from enum import Enum
class Solution:
    def isNumber(self, s: str) -> bool:
        State = Enum('State', (
            'START', 'SYM_START_DECIMAL_PNT', 'SYM_MID_DECIMAL_PNT', 'NUM_SIMPLE',
            'NUM_FIRST_PART', 'NUM_SECOND_PART', 'SYM_SCI', 'NUM_AFT_SCI', 
            'SYM_PN_SIMPLE', 'SYM_PN_SCI', 'ERROR'))
        end_state = (
            State.NUM_SIMPLE, State.SYM_MID_DECIMAL_PNT, State.NUM_FIRST_PART, 
            State.NUM_SECOND_PART, State.NUM_AFT_SCI)

        now_state = State.START
        s = s.strip()
        s_iter = iter(s)
        while True:
            if now_state == State.ERROR:
                return False
            
            try:
                now_chr = next(s_iter)
            except StopIteration:
                if now_state in end_state:
                    return True
                else:
                    return False
            
            if now_state == State.START:
                if now_chr.isnumeric():
                    now_state = State.NUM_FIRST_PART
                    continue
                elif now_chr in ['+', '-']:
                    now_state = State.SYM_PN_SIMPLE
                    continue
                elif now_chr == ".":
                    now_state = State.SYM_START_DECIMAL_PNT
                    continue
            elif now_state == State.NUM_FIRST_PART:
                if now_chr.isnumeric():
                    continue
                elif now_chr == ".":
                    now_state = State.SYM_MID_DECIMAL_PNT
                    continue
                elif now_chr in ["e", "E"]:
                    now_state = State.SYM_SCI
                    continue
            elif now_state == State.SYM_START_DECIMAL_PNT:
                if now_chr.isnumeric():
                    now_state = State.NUM_SIMPLE
                    continue
            elif now_state == State.SYM_MID_DECIMAL_PNT:
                if now_chr.isnumeric():
                    now_state = State.NUM_SECOND_PART
                    continue
                elif now_chr in ["e", "E"]:
                    now_state = State.SYM_SCI
                    continue
            elif now_state == State.NUM_SIMPLE:
                if now_chr in ["e", "E"]:
                    now_state = State.SYM_SCI
                    continue
                elif now_chr.isnumeric():
                    continue
            elif now_state == State.NUM_SECOND_PART:
                if now_chr.isnumeric():
                    continue
                elif now_chr in ["e", "E"]:
                    now_state = State.SYM_SCI
                    continue
            elif now_state == State.SYM_SCI:
                if now_chr.isnumeric():
                    now_state = State.NUM_AFT_SCI
                    continue
                elif now_chr in ["+", "-"]:
                    now_state = State.SYM_PN_SCI
                    continue
            elif now_state == State.NUM_AFT_SCI:
                if now_chr.isnumeric():
                    continue
            elif now_state == State.SYM_PN_SIMPLE:
                if now_chr == ".":
                    now_state = State.SYM_START_DECIMAL_PNT
                    continue
                elif now_chr.isnumeric():
                    now_state = State.NUM_FIRST_PART
                    continue
            elif now_state == State.SYM_PN_SCI:
                if now_chr.isnumeric():
                    now_state = State.NUM_AFT_SCI
                    continue
            now_state = State.ERROR