feat: add [509] 斐波那契数

Author: pdsuwwz

509.斐波那契数.ts

@@ -0,0 +1,37 @@
+/*
+ * @lc app=leetcode.cn id=509 lang=typescript
+ *
+ * [509] 斐波那契数
+ */
+
+// @lc code=start
+function fib(n: number): number {
+  
+  const cacheMapping = new Map<number, number>()
+  
+  const compute = (n: number) => {
+    if (cacheMapping.has(n)) return cacheMapping.get(n)
+    if (n === 0) {
+      cacheMapping.set(n, 0)
+      return 0
+    }
+    if (n <= 2) {
+      cacheMapping.set(n, 1)
+      return 1
+    }
+    
+    const prev = compute(n - 2)
+    const next = compute(n - 1)
+
+    cacheMapping.set(n - 2, prev)
+    cacheMapping.set(n - 1, next)
+    
+    return prev + next
+  }
+
+  return compute(n)
+};
+// @lc code=end
+
+// 0 1 2 3 4
+// 0 1 1 2 3 5