Merge pull request #245 from EvanLjp/master

reapired the mistake ,which is the regular expression can inut a nill…

notes/剑指 offer 题解.md

@@ -1017,6 +1017,7 @@ public boolean match(char[] str, char[] pattern) {
 
 ```java
 public boolean isNumeric(char[] str) {
+    if(str.length==0||str==null) return false;
     return new String(str).matches("[+-]?\\d*(\\.\\d+)?([eE][+-]?\\d+)?");
 }
 ```