Valid Number

Validate if a given string is numeric.

Some examples:
"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

class Solution {
public:
    bool isNumber(string s) {
        int len = s.length();
        int dotNum = 0;
        int eNum = 0;

        //去掉首位的空格
        int start = len;
        for(int i=0;i<len;i++){
            if(s[i] != ' '){
                start = i;
                break;
            }
        }
        for(int i=len-1;i>=0;i--){
            if(s[i] == ' '){
                len--;
            }else{
                break;
            }
        }
        if(start > len) return false;

        for(int i=start;i<len;i++){
            if(s[i] >= '0' && s[i] <= '9'){
                continue;
            }else if(s[i] == '.'){
                if(dotNum > 0 || eNum >0 ) return false;
                dotNum++;
                if(i == start && i == len-1) return false;
                if(!(s[i-1] >= '0' && s[i-1] <= '9') && (i==len-1 || !(s[i+1] >= '0' && s[i+1] <= '9'))) return false;
            }else if(s[i] == 'e'){
                if(eNum > 0) return false;
                eNum++;
                if(i==start) return false;
                if(s[i-1] == '-' || s[i-1] == '+')  return false;
                if( i == len - 1)  return false;
            }else if(s[i] == '-'){
                if(i != start && s[i-1] != 'e') return false;
                if(i == len-1) return false;
            }else if(s[i] == '+'){
                if(i != start && s[i-1] != 'e') return false;
                if(i==len-1) return false;
            }else{
                return false;
            }
        }
        return true;

    }
};