leetcode - Valid Number

class Solution {
public:
bool isInt(const char * s, int start, int end){
if (end < start)
return false;
if  (start<=end && (s[start] == '+' || s[start] == '-'))
start++;
if (end<start)
return false;
for (int i = start; i <= end; i++){
if (s[i]<'0' || s[i]>'9')
return false;
}
return true;
}
bool isDouble(const char * s, int start, int end){
if (start <= end && (s[start] == '+' || s[start] == '-'))
start++;
for (int i = start; i <= end; i++){
if (s[i] == '.'){
if (i + 1 <= end && (s[i + 1] == '-' || s[i + 1] == '+'))
return false;
if (i == start)
return isInt(s, i + 1, end);
else if (i == end)
return isInt(s, start, i-1);
else return isInt(s, start, i - 1) && isInt(s, i + 1, end);
}
}
return isInt(s, start, end);
}
bool isNumber(const char *s) {
// Note: The Solution object is instantiated only once and is reused by each test case.
int len = strlen(s);
int start = 0, end = len - 1;
while (start <= end && (s[start] == ' '))
start++;
while (end >= start && s[end] == ' ')
end--;
for (int i = start; i <= end; i++){
if (s[i] == 'e' || s[i] == 'E'){
return isDouble(s, start, i - 1) && isInt(s, i + 1, end);
}
}
return isDouble(s, start , end);
}
};