509 斐波那契数列 Python 3 解法

动态规划系列/动态规划详解进阶.md

@@ -366,4 +366,45 @@ PS：为啥 `dp` 数组初始化为 `amount + 1` 呢，因为凑成 `amount` 金
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-======其他语言代码======
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+======其他语言代码======
+
+[DapangLiu](https://github.com/DapangLiu) 提供 509. 斐波那契数 Python3 解法代码：
+
+递归写法
+
+```python
+class Solution:
+    def fib(self, N: int) -> int:
+        if N <= 1:
+            return N
+        return self.fib(N-1) + self.fib(N-2)
+```
+
+动态规划写法
+
+```python
+class Solution:
+    def fib(self, N: int) -> int:
+        if N == 0:
+            return 0
+        # init
+        result = [0 for i in range(N+1)]
+        result[1] = 1
+
+        # status transition
+        for j in range(2, N+1):
+            result[j] = result[j-1] + result[j-2]
+        return result[-1]
+```
+
+动态规划写法 (状态压缩)
+
+```python
+class Solution:
+    def fib(self, n: int) -> int:
+        # current status only depends on two previous status
+        dp_0, dp_1 = 0, 1
+        for _ in range(n):
+            dp_0, dp_1 = dp_1, dp_0 + dp_1
+        return dp_0
+```
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