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. http://leetcode.com/onlinejudge#question_65

leetcode_valid_number.cpp

class Solution {
public:
    bool isInt(const char* s, int len) {
        if (len == 0)
            return false;
        int st = 0;
        if (s[0] == '+' || s[0] == '-')
            st = 1;
        if (st == len)
            return false;
        for (int i = st; i < len; ++i)
            if (s[i] >= '0' && s[i] <= '9')
                ;
            else
                return false;
        return true;
    }

    bool isNumber__(const char* s, int len) {
        int dot_pos = -1;
        for (int i = 0; i < len; ++i)
            if (s[i] == '.') {
                if (dot_pos == -1)
                    dot_pos = i;
                else
                    return false;
            }
        if (dot_pos == -1)
            return isInt(s, len);
        else {
            if (len == 1)
                return false;
            if (dot_pos != len - 1) {
                if (s[dot_pos + 1] >= '0' && s[dot_pos + 1] <= '9') {
                    if (dot_pos != len - 2 && !isInt(s + dot_pos + 1, len - dot_pos - 1))
                        return false;
                }
                else
                    return false;
            }
            if (dot_pos != 0) {
                if (dot_pos == 1 && (s[0] == '+' || s[0] == '-')) {
                    if (len == 2)
                        return false;
                    else
                        return true;
                }
                if (!isInt(s, dot_pos))
                    return false;
            }
            return true;
        }
    }

    bool isNumber_(const char* s, int len) {
        int e_pos = -1;
        for (int i = 0; i < len; ++i)
            if (s[i] == 'e') {
                if (e_pos == -1)
                    e_pos = i;
                else
                    return false;
            }
        if (e_pos == 0 || e_pos == len - 1)
            return false;
        if (e_pos != -1)
            return isNumber__(s, e_pos) && isInt(s + e_pos + 1, len - e_pos - 1);
        else
            return isNumber__(s, len);
    }

    bool isNumber(const char *s) {
        int len = strlen(s);
        int end = len;
        for (int i = len - 1; i >= 0; --i)
            if (s[i] == ' ' || s[i] == '\t')
                end = i;
            else
                break;
        for (int i = 0; s[i] != '\0'; ++i)
            if (s[i] != ' ' && s[i] != '\t')
                return isNumber_(s + i, end - i);
        return false;
    }
};