Update Fibonacci Number - Leetcode 509.py

Author: gahogg

Fibonacci Number - Leetcode 509.py

@@ -1 +1,71 @@
-test
+# Top Down Recursive
+
+# Time: O(2^n)
+# Space: O(n)
+class Solution:
+    def fib(self, n: int) -> int:
+        if n == 0:
+            return 0
+        elif n == 1:
+            return 1
+        else:
+            return self.fib(n-1) + self.fib(n-2)
+
+
+# Top Down Memoized
+
+# Time: O(n)
+# Space: O(n)
+class Solution:
+    def fib(self, n: int) -> int:
+        memo = {0:0, 1:1}
+
+        def f(x):
+            if x in memo:
+                return memo[x]
+            else:
+                memo[x] = f(x-1) + f(x-2)
+                return memo[x]
+        
+        return f(n)
+
+
+# Bottom Up Tabulation
+
+# Time: O(n)
+# Space: O(n)
+class Solution:
+    def fib(self, n: int) -> int:
+        if n == 0:
+            return 0
+        if n == 1:
+            return 1
+
+        dp = [0] * (n+1)
+        dp[0] = 0
+        dp[1] = 1
+
+        for i in range(2, n+1):
+            dp[i] = dp[i-2] + dp[i-1]
+
+        return dp[n]
+
+
+# Bottom Up Constant Space
+
+# Time: O(n)
+# Space: O(1)
+class Solution:
+    def fib(self, n: int) -> int:
+        if n == 0:
+            return 0
+        if n == 1:
+            return 1
+
+        prev = 0
+        cur = 1
+
+        for i in range(2, n+1):
+            prev, cur = cur, prev+cur
+        
+        return cur