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Commit fdf4bc0

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Merge pull request #4 from borrelunde/fibonacci_number
LeetCode problem: 509. Fibonacci Number
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# 509. Fibonacci Number
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Difficulty: `Easy`
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Topics: `Math`, `Dynamic Programming`, `Recursion`, `Memoization`
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The **Fibonacci numbers**, commonly denoted `F(n)` form a sequence, called the **Fibonacci sequence**, such that each
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number is the sum of the two preceding ones, starting from `0` and `1`. That is,
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```text
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F(0) = 0, F(1) = 1
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F(n) = F(n - 1) + F(n - 2), for n > 1.
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```
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Given `n`, calculate `F(n)`.
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**Example 1:**
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```text
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Input: n = 2
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Output: 1
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Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
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```
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**Example 2:**
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```text
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Input: n = 3
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Output: 2
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Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
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```
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**Example 3:**
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```text
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Input: n = 4
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Output: 3
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Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
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```
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**Constraints:**
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- `0 <= n <= 30`
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package com.bl.fibonacci_number;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* This is the solution to the LeetCode problem: 509. Fibonacci Number
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*
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* @author Børre A. Opedal Lunde
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* @since 2024年01月27日
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*/
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public class Solution {
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// The cache of Fibonacci numbers.
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private static final List<Integer> FIBONACCI_NUMBERS_CACHE = new ArrayList<>();
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// Initialize the cache with the two first Fibonacci numbers.
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static {
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FIBONACCI_NUMBERS_CACHE.add(0);
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FIBONACCI_NUMBERS_CACHE.add(1);
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}
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public int fib(int n) {
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final int fibonacciNumber;
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final int lastCachedFibonacciNumberIndex = FIBONACCI_NUMBERS_CACHE.size() - 1;
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// Check if the Fibonacci number is already cached.
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if (n <= lastCachedFibonacciNumberIndex) {
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// Retrieve the Fibonacci number from the cache.
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fibonacciNumber = FIBONACCI_NUMBERS_CACHE.get(n);
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// Return the Fibonacci number.
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return fibonacciNumber;
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}
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// If the Fibonacci number is not in the cache, let's start from where
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// we left off and put it there.
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// Retrieve the two last Fibonacci numbers from the cache.
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final int a = FIBONACCI_NUMBERS_CACHE.get(lastCachedFibonacciNumberIndex - 1);
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final int b = FIBONACCI_NUMBERS_CACHE.get(lastCachedFibonacciNumberIndex);
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// Calculate the Fibonacci number.
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fibonacciNumber = a + b;
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// Add the Fibonacci number to the cache.
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FIBONACCI_NUMBERS_CACHE.add(fibonacciNumber);
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// Check if we have reached the Fibonacci number we are looking for.
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if (n - 1 == lastCachedFibonacciNumberIndex) {
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return fibonacciNumber;
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}
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// If not, let's continue recursively until we reach it.
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return fib(n);
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}
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}
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package com.bl.fibonacci_number;
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import org.junit.jupiter.api.DisplayName;
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import org.junit.jupiter.api.Test;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.Arguments;
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import org.junit.jupiter.params.provider.MethodSource;
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import java.util.stream.Stream;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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/**
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* This is the test to the LeetCode problem: 509. Fibonacci Number
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*
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* @author Børre A. Opedal Lunde
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* @since 2024年01月27日
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*/
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@DisplayName("Fibonacci Number")
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class SolutionTest {
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final Solution solution = new Solution();
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@Test
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@DisplayName("Example one")
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void exampleOne() {
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int n = 2;
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int expected = 1;
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int actual = solution.fib(n);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Example two")
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void exampleTwo() {
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int n = 3;
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int expected = 2;
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int actual = solution.fib(n);
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assertEquals(expected, actual);
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}
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@Test
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@DisplayName("Example three")
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void exampleThree() {
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int n = 4;
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int expected = 3;
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int actual = solution.fib(n);
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assertEquals(expected, actual);
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}
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static Stream<Arguments> provideFibonacciNumbersFromZeroToThirty() {
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return Stream.of(
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Arguments.of(0, 0),
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Arguments.of(1, 1),
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Arguments.of(2, 1),
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Arguments.of(3, 2),
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Arguments.of(4, 3),
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Arguments.of(5, 5),
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Arguments.of(6, 8),
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Arguments.of(7, 13),
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Arguments.of(8, 21),
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Arguments.of(9, 34),
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Arguments.of(10, 55),
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Arguments.of(11, 89),
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Arguments.of(12, 144),
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Arguments.of(13, 233),
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Arguments.of(14, 377),
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Arguments.of(15, 610),
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Arguments.of(16, 987),
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Arguments.of(17, 1597),
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Arguments.of(18, 2584),
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Arguments.of(19, 4181),
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Arguments.of(20, 6765),
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Arguments.of(21, 10946),
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Arguments.of(22, 17711),
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Arguments.of(23, 28657),
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Arguments.of(24, 46368),
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Arguments.of(25, 75025),
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Arguments.of(26, 121393),
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Arguments.of(27, 196418),
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Arguments.of(28, 317811),
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Arguments.of(29, 514229),
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Arguments.of(30, 832040)
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// Stop at 30 because the problem description
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// uses that as its upper limit.
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);
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}
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@ParameterizedTest(name = "n {0} is Fibonacci number {1}")
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@DisplayName("Fibonacci numbers from zero to thirty")
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@MethodSource("provideFibonacciNumbersFromZeroToThirty")
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void fibonacciNumbersFromZeroToThirty(int n, int expected) {
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int actual = solution.fib(n);
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assertEquals(expected, actual);
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}
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}

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