Sword finger offer 20 String representing numeric value

Use numbers , The sign , spot , size e The mutually exclusive relationship between , Simplified state machine , To solve the , Learned ！

class Solution {
    
public:
    bool isNumber(string s) {
    
        if(s.size() == 0) return false;

        // Skip leading and trailing spaces 
        int left = 0, right = s.length() - 1;
        while(left <= right && s[left] == ' '){
    
            left++;
        }// The beginning of the string 
        if(left > right) return false;// Exclude strings that are all spaces 
        while(left < right && s[right] == ' '){
    
            right--;
        }// End of string 

        bool isNum = false;
        bool isDot = false;
        bool isEe = false;
        bool isSign = false;

        for(int i = left; i <= right; i++){
    // Left closed right closed interval 
            if(s[i] >= '0' && s[i] <= '9'){
    
                isNum = true;
            }
            //  One '.';e/E Followed by an integer （ Can not have '.'）
            else if(s[i] == '.' && !isDot && !isEe){
    
                isDot = true;
            }
            //  One 'E' or 'e'; Numbers need to appear before 
            else if((s[i] == 'E' || s[i] == 'e') && isNum && !isEe){
    
                isEe = true;
                  avoid e The situation at the end  e It must be followed by an integer 
                isNum = false;
            }
            // '+''-' It can only appear at the beginning or 'E' or 'e' The last one of them 
            else if((s[i] == '+' || s[i] == '-') && (i == left || s[i - 1] == 'E' || s[i - 1] == 'e')){
    
                isSign = true;
            }
            else{
    
                return false;
            }
        }
        //  Must end with a number 
        return isNum;
    }
};