Valid Number

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.

---------------------------------------- thinking -----------------------------------
https://leetcode.com/discuss/24878/very-simple-c-sol-not-a-dfa-sol

First let me talk about some requirements.

1. There could be unlimited space before and after number.
2. There could be one '+' or '-' before number.
3. (left).(right) The expression before or after dot could be null, but not both.
4. (left)e(+/-)(right)
   (1) The expression before e could be a float number.
   (2) The right expression must be a positive integer.
   (3) Both left and right expression couldn't be null.
   (4) There could be a '+' or '-' after e.

------------------------------------------- codes --------------------------------------------

class Solution {

public:

    bool isNumber(string s) {

        int ind = 0;

        skipSpace(s, ind);

        skipSign(s, ind);

        if (!isNumber(s, ind)) {

            return false;

        }

        if (ind < s.size() && (s[ind] == 'e'|| s[ind] == 'E')) {

            ind++;

            skipSign(s, ind);

            if (!isInteger(s, ind)) {

                return false;

            }

        }

        skipSpace(s, ind);

        if (ind < s.size()) {

            return false;

        } else {

            return true;

        }

    }

    bool skipSpace(string s, int &ind) {

        int len = 0;

        while (ind < s.size() && s[ind] == ' ') {

            ind++;

            len++;

        }

        if (len > 0) {

            return true;

        } else {

            return false;

        }

    }

    bool skipSign(string s, int &ind) {

        if (ind < s.size() && (s[ind] == '+' || s[ind] == '-')) {

            ind++;

            return true;

        } else {

            return false;

        }

    }

    bool isInteger(string s, int &ind) {

        int len = 0;

        while (ind < s.size() && (s[ind] >= '0' && s[ind] <= '9')) {

            ind++;

            len++;

        }

        if (len > 0) {

            return true;

        } else {

            return false;

        }

    }

    bool isNumber(string s, int &ind) {

        int len = 0;

        while (ind < s.size() && (s[ind] >= '0' && s[ind] <= '9')) {

            ind++;

            len++;

        }

        if (len>0 && ind == s.size()) {

            return true;

        }

        if (s[ind] == '.') {

            ind++;

        } 

        while (ind < s.size() && (s[ind] >= '0' && s[ind] <= '9')) {

            ind++;

            len++;

        }

        if (len > 0) {

            return true;

        } else {

            return false;

        }

    }

};