Update 0509.斐波那契数.md

problems/0509.斐波那契数.md

@@ -230,17 +230,41 @@ class Solution:
         return dp[n]
 
 ```
-动态规划（版本二）
+态规划（版本二）
 ```python 
+
 class Solution:
     def fib(self, n: int) -> int:
-        if n < 2:
+        if n <= 1:
             return n
-        a, b, c = 0, 1, 0
-        for i in range(1, n):
-            c = a + b
-            a, b = b, c
-        return c
+
+        dp = [0, 1]
+
+        for i in range(2, n + 1):
+            total = dp[0] + dp[1]
+            dp[0] = dp[1]
+            dp[1] = total
+
+        return dp[1]
+
+
+```
+动态规划（版本三）
+```python 
+class Solution:
+    def fib(self, n: int) -> int:
+        if n <= 1:
+            return n
+
+        prev1, prev2 = 0, 1
+
+        for _ in range(2, n + 1):
+            curr = prev1 + prev2
+            prev1, prev2 = prev2, curr
+
+        return prev2
+
+
 
 
 ```