# Valid Number

Hard

## Question

Validate if a given string is numeric.

Example
“0” => true
” 0.1 “ => true
“abc” => false
“1 a” => false
“2e10” => true

## Thinking

This question is not hard. The main thing is that you need to clarify what is a numeric since the question is no clear.

• Empty string - false
• Single character “.”, “+”, “-“ - false
• Starts with “e” or ends with “e” - false
• “e” must behind dot if dot exists.
• only one “e” and one “.”
• ”+”, “-“ must be the first character if they exist.

## Solution

#### Java

``````public class Solution {
/**
* @param s the string that represents a number
* @return whether the string is a valid number
*/
public boolean isNumber(String s) {
if (s == null) {
return false;
}
s = s.trim();
// special case only one dot or empty string
if (s.length() == 0 || s.equals(".") || s.equals("+") || s.equals("-")) {
return false;
}
// special cases (start or end with 'e')
if (s.charAt(0) == 'e' || s.charAt(s.length() - 1) == 'e') {
return false;
}
boolean hasDot = false;
boolean hasE = false;
for (int i = 0; i < s.length(); i++) {
char cstr = s.charAt(i);
if (cstr == '.') {
if (hasDot || hasE) { // e must behind dot
return false;
}
hasDot = true;
} else if (cstr == 'e') {
if (hasE) {
return false;
}
hasE = true;
} else if (cstr >= '0' && cstr <= '9') {
continue;
} else if (cstr == '+' || cstr == '-') {
if (i > 0) {
return false;
}
} else {
return false;
}
}
return true;
}
}
``````