# 0065. Valid Number
###### tags: `Leetcode` `Hard` `FaceBook`
Link: https://leetcode.com/problems/valid-number/
## 思路 $O(N)$ $O(1)$
1. Declare 3 variables $seenDigit$, $seenExponent$, and $seenDot$. Set all of them to $false$.
2. Iterate over the input.
3. If the character is a digit, set $seenDigit = true$.
4. If the character is a sign, check if it is either the first character of the input, or if the character before it is an exponent. If not, return $false$.
5. If the character is an exponent, first check if we have already seen an exponent or if we have not yet seen a digit. If either is $true$, then return $false$. Otherwise, set $seenExponent = true$, and $seenDigit = false$. We need to reset $seenDigit$ because after an exponent, we must construct a new integer.
6. If the character is a dot, first check if we have already seen either a dot or an exponent. If so, return $false$. Otherwise, set $seenDot = true$.
7. If the character is anything else, return $false$.
8. At the end, return $seenDigit$. This is one reason why we have to reset $seenDigit$ after seeing an exponent - otherwise an input like "21e" would be incorrectly judged as valid.
## Code

class Solution {
    public boolean isNumber(String s) {
        boolean seenDigit = false;
        boolean seenExponent = false;
        boolean seenDot = false;
        for(int i = 0;i < s.length();i++){
            char curr = s.charAt(i);
            if(Character.isDigit(curr)){
                seenDigit = true;
            }
            else if(curr=='+' || curr=='-'){
                if(i>0 && s.charAt(i-1)!='E' && s.charAt(i-1)!='e'){
                    return false;
                }
            }
            else if(curr=='E' || curr=='e'){
                if(seenExponent || !seenDigit) return false;
                seenExponent = true;
                seenDigit = false;
            }
            else if(curr=='.'){
                if(seenDot || seenExponent) return false;
                seenDot = true;
            }
            else{
                return false;
            }
        }
        return seenDigit;
    }
}