Valid_Number.cpp

class Solution {
public:
bool isNumber(string s) {
if(s.length() == 0) return false;
 int i = 0,n = s.length()-1;
bool e = false, dot = false, digit = false;

while(i < s.length() && s[i] == ' ') i++;    // removing white spaces
while(n>= 0 && s[n] == ' ') n--;

if(s[i] == '+' || s[i] == '-') i++;
if(i > n) return false;
for(; i <= n; i++){
	if(isdigit(s[i])){
		digit = true;     // atleast one digit
		continue;
	}
	else if(s[i] == 'e' && digit && i != n && !e){  //atmost one exponent with atleast one digit
	    e = true;
		if(s[i + 1] == '+' || s[i + 1] == '-')     // sign is only allowed after 'e'
			if(i == n - 1) return false;
			else i++;
	}
	else if(s[i] == '.' && !dot && !e)  dot = true;     // atmost one dot
	else return false;
}
return digit;
    }
};