solve 509: fibonacci number

Author: andavid

vscode/509.fibonacci-number.java

@@ -0,0 +1,77 @@
+/*
+ * @lc app=leetcode id=509 lang=java
+ *
+ * [509] Fibonacci Number
+ *
+ * https://leetcode.com/problems/fibonacci-number/description/
+ *
+ * algorithms
+ * Easy (66.65%)
+ * Total Accepted:    39.3K
+ * Total Submissions: 58.7K
+ * Testcase Example:  '2'
+ *
+ * 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: 2
+ * Output: 1
+ * Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: 3
+ * Output: 2
+ * Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
+ * 
+ * 
+ * Example 3:
+ * 
+ * 
+ * Input: 4
+ * Output: 3
+ * Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
+ * 
+ * 
+ * 
+ * 
+ * Note:
+ * 
+ * 0 ≤ N ≤ 30.
+ * 
+ */
+class Solution {
+  public int fib(int n) {
+    if (n <= 0) return 0;
+    if (n == 1) return 1;
+    if (n == 2) return 1;
+
+    int one_step_before = 1;
+    int two_steps_before = 1;
+    int result = 0;
+
+    for (int i = 2; i < n; i++) {
+      result = one_step_before + two_steps_before;
+      two_steps_before = one_step_before;
+      one_step_before = result;
+    }
+    
+    return result;
+  }
+}
+