Update 0509.斐波那契数.md

problems/0509.斐波那契数.md

@@ -171,7 +171,20 @@ public:
 
 
 Java：
-
+```Java
+class Solution {
+    public int fib(int n) {
+        if (n < 2) return n;
+        int a = 0, b = 1, c = 0;
+        for (int i = 1; i < n; i++) {
+            c = a + b;
+            a = b;
+            b = c;
+        }
+        return c;
+    }
+}
+```
 
 Python：