Fibonacci number.

Author: recherst

Easy/509.Fibonacci Number.playground/Contents.swift

@@ -0,0 +1,45 @@
+/**
+ The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,
+
+ F(0) = 0, F(1) = 1
+ F(n) = F(n - 1) + F(n - 2), for n > 1.
+ Given n, calculate F(n).
+
+  
+
+ Example 1:
+ Input: n = 2
+ Output: 1
+ Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
+ 
+ Example 2:
+ Input: n = 3
+ Output: 2
+ Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
+ 
+ Example 3:
+ Input: n = 4
+ Output: 3
+ Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
+  
+
+ Constraints:
+ - 0 <= n <= 30
+ */
+class Solution {
+    func fib(_ n: Int) -> Int {
+        if n == 1 { return 1 }
+        if n == 0 { return 0 }
+        var sum = [0, 1]
+        var current = 0
+        for i in 2...n {
+            current = sum[i - 1] + sum[i - 2]
+            sum.append(current)
+        }
+        return sum.last!
+    }
+}
+
+let s = Solution()
+let r = s.fib(4)
+print(r)

Easy/509.Fibonacci Number.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios' buildActiveScheme='true' importAppTypes='true'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

Easy/509.Fibonacci Number.playground/playground.xcworkspace/contents.xcworkspacedata

@@ -104,6 +104,7 @@
 100. [Base 7](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/504.Base%207.playground/Contents.swift)
 101. [Relative Ranks](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/506.Relative%20Ranks.playground/Contents.swift)
 102. [Perfect Number](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/507.Perfect%20Number.playground/Contents.swift)
+103. [Fibonacci Number](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/509.Fibonacci%20Number.playground/Contents.swift)
 
 #### Medium