LeetCode 剑指 Offer 20. 表示数值的字符串

剑指 Offer 20. 表示数值的字符串

请实现一个函数用来判断字符串是否表示数值（包括整数和小数）。例如，字符串"+100"、“5e2”、"-123"、“3.1416”、"-1E-16"、“0123"都表示数值，但"12e”、“1a3.14”、“1.2.3”、"±5"及"12e+5.4"都不是。

思路

有限状态自动机。设计状态的时候注意由于".1"这样的字符串是合法的而"."这样的字符串是非法的，因此针对’.‘要设计两个状态: DOT, DOT_NO_DIG. 前者表示’.‘之前已有数字，后者表示’.'之前没有数字。另外注意字符串前后可能含有空格，但字符串前后的空格不影响字符串的含义（字符串中间的空格会使得字符串非法），因此要提前用strip或trim方法对字符串进行预处理。

代码

class Solution {
     
    private static final short START = -1, SIGN_A = 0, DIG_A0 = 1, DOT = 2, DOT_NO_DIG = 3, DIG_A1 = 4, E = 5, SIGN_B = 6, DIG_B = 7;

    private short getCharType(char ch) {
     
        if (ch >= '0' && ch <= '9') {
     
            return 0;
        } else if (ch == '+' || ch == '-') {
     
            return 1;
        } else if (ch == '.') {
     
            return 2;
        } else if (ch == 'E' || ch == 'e') {
     
            return 3;
        } else {
     
            return -1;
        }
    }

    public boolean isNumber(String s) {
     
        s = s.strip();
        short state = START;
        for (char ch: s.toCharArray()) {
     
            short type = getCharType(ch);
            if (type == -1) {
     
                return false;
            }
            switch (state) {
     
                case START:
                    switch (type) {
     
                        case 0:
                            state = DIG_A0;
                            break;
                        case 1:
                            state = SIGN_A;
                            break;
                        case 2:
                            state = DOT_NO_DIG;
                            break;
                        default:
                            return false;
                    }
                    break;
                case SIGN_A:
                    switch (type) {
     
                        case 0:
                            state = DIG_A0;
                            break;
                        case 2:
                            state = DOT_NO_DIG;
                            break;
                        default:
                            return false;
                    }
                    break;
                case DIG_A0:
                    switch (type) {
     
                        case 0:
                            state = DIG_A0;
                            break;
                        case 2:
                            state = DOT;
                            break;
                        case 3:
                            state = E;
                            break;
                        default:
                            return false;
                    }
                    break;
                case DOT:
                    switch (type) {
     
                        case 0:
                            state = DIG_A1;
                            break;
                        case 3:
                            state = E;
                            break;
                        default:
                            return false;
                    }
                    break;
                case DOT_NO_DIG:
                    switch (type) {
     
                        case 0:
                            state = DIG_A1;
                            break;
                        default:
                            return false;
                    }
                    break;
                case DIG_A1:
                    switch (type) {
     
                        case 0:
                            state = DIG_A1;
                            break;
                        case 3:
                            state = E;
                            break;
                        default:
                            return false;
                    }
                    break;
                case E:
                    switch (type) {
     
                        case 0:
                            state = DIG_B;
                            break;
                        case 1:
                            state = SIGN_B;
                            break;
                        default:
                            return false;
                    }
                    break;
                case SIGN_B:
                    switch (type) {
     
                        case 0:
                            state = DIG_B;
                            break;
                        default:
                            return false;
                    }
                    break;
                case DIG_B:
                    switch (type) {
     
                        case 0:
                            state = DIG_B;
                            break;
                        default:
                            return false;
                    }
                    break;
                default:
                    return false;
            }
        }
        return state != START && state != DOT_NO_DIG && state != E && state != SIGN_A && state != SIGN_B;
    }
}