cpp_src/ValidNumber.cpp

/**
 * This is the worst description I have ever met in LeetCode.
 * The solution is from https://leetcode.com/discuss/9013/a-simple-solution-in-cpp
 */
class Solution {
public:
    bool isNumber(string s) {
        int i = 0;
        // skip the whilespaces
        for(; s[i] == ' '; i++) {}

        // check the significand
        if(s[i] == '+' || s[i] == '-') i++; // skip the sign if exist

        int n_nm, n_pt;
        for(n_nm=0, n_pt=0; (s[i]<='9' && s[i]>='0') || s[i]=='.'; i++)
            s[i] == '.' ? n_pt++:n_nm++;       
        if(n_pt>1 || n_nm<1) // no more than one point, at least one digit
            return false;

        // check the exponent if exist
        if(s[i] == 'e') {
            i++;
            if(s[i] == '+' || s[i] == '-') i++; // skip the sign

            int n_nm = 0;
            for(; s[i]>='0' && s[i]<='9'; i++, n_nm++) {}
            if(n_nm<1)
                return false;
        }

        // skip the trailing whitespaces
        for(; s[i] == ' '; i++) {}

        return s[i]==0;  // must reach the ending 0 of the string
    }
};