Valid Number

Author: mengli

ValidNumber.java

@@ -0,0 +1,85 @@
+/**
+ * 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.
+ *
+ */
+public class ValidNumber {
+	public boolean isNumber(String s) {
+		StringBuffer sb = new StringBuffer(s.trim());
+		int i = sb.length() - 1;
+		while (i >= 0 && sb.charAt(i) == ' ') {
+			i--;
+		}
+		sb.delete(i + 1, sb.length());
+		s = sb.toString();
+		int length = s.length();
+		if (length == 0)
+			return false;
+		i = 0;
+		int start = 0;
+		if (s.charAt(i) == '-' || s.charAt(i) == '+') {
+			i++;
+			start++;
+		}
+		for (; i < length; i++) {
+			char c = s.charAt(i);
+			if (c == 'e' || c == 'E') {
+				return isDouble(s.substring(start, i)) && isDigitals(s.substring(i + 1, s.length()));
+			} else if (c != '.' && (c < '0' || c > '9')) {
+				return false;
+			}
+		}
+		return isDouble(s.substring(start));
+	}
+
+	private boolean isDouble(String s) {
+		int length = s.length();
+		if (length == 0 || ((s.charAt(0) == '-' || s.charAt(0) == '+') && length == 1))
+			return false;
+		int i = 0, start = 0;
+		if (s.charAt(i) == '-' || s.charAt(i) == '+') {
+			i++;
+			start++;
+		}
+		for (; i < length; i++) {
+			char c = s.charAt(i);
+			if (c == '.') {
+				if (i == start && s.length() - i - 1 == 0)
+					return false;
+				boolean left = i == start ? true : isDigitals(s.substring(0, i));
+				boolean right = s.length() - i - 1 == 0 ? true : isDigitals(s.substring(i + 1, s.length()));
+				return left && right;
+			} else if (c < '0' || c > '9') {
+				return false;
+			}
+		}
+		return true;
+	}
+
+	private boolean isDigitals(String s) {
+		int length = s.length();
+		if (length == 0 || ((s.charAt(0) == '-' || s.charAt(0) == '+') && length == 1))
+			return false;
+		int i = 0;
+		if (s.charAt(i) == '-' || s.charAt(i) == '+') {
+			i++;
+		}
+		for (; i < length; i++) {
+			char c = s.charAt(i);
+			if (c < '0' || c > '9') {
+				return false;
+			}
+		}
+		return true;
+	}
+}