leetcode——65. Valid Number

A valid number can be split up into these components (in order):

1. A decimal number or an integer.
2. (Optional) An ‘e’ or ‘E’, followed by an integer.

A decimal number can be split up into these components (in order):

1. (Optional) A sign character (either ‘+’ or ‘-’).
2. One of the following formats:
   1. One or more digits, followed by a dot ‘.’.
   2. One or more digits, followed by a dot ‘.’, followed by one or more digits.
   3. A dot ‘.’, followed by one or more digits.

An integer can be split up into these components (in order):

1. (Optional) A sign character (either ‘+’ or ‘-’).
2. One or more digits.

For example, all the following are valid numbers: [“2”, “0089”, “-0.1”, “+3.14”, “4.”, “-.9”, “2e10”, “-90E3”, “3e+7”, “+6e-1”, “53.5e93”, “-123.456e789”],

while the following are not valid numbers: [“abc”, “1a”, “1e”, “e3”, “99e2.5”, “–6”, “-+3”, “95a54e53”].

Given a string s, return true if s is a valid number.

class Solution {
public:
    bool isNumber(string s) {
    	int n = s.size();
    	bool num_flag = false, dot_flag = false, e_flag = false;
    	for(int i=0; i<n; i++){
    		if(s[i] >= '0' && s[i] <= '9'){
    			num_flag = true;
    		}
    		else if(s[i] == '.' && !dot_flag && !e_flag){
    			dot_flag = true;
    		}
    		else if((s[i] == 'e' || s[i] == 'E') && !e_flag && num_flag){
    			e_flag = true;
    			num_flag = false;
    		}
    		else if((s[i] == '+' || s[i] == '-') && (i == 0 || (s[i-1] == 'e'||s[i-1] == 'E'))){
    			continue;
    		}
    		else 
    			return false;
    	}
    	return num_flag;
    }
};