FibonacciNumber.java

package easy;
/**
 * ClassName: FibonacciNumber.java
 * Author: chenyiAlone
 * Create Time: 2019/9/25 22:15
 * Description: No.509 Fibonacci Number
 * 思路:
 *      递推
 *
 * 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.
 *
 *
 */
public class FibonacciNumber {
    
    public int fib(int N) {
        int pre = 0, ret = 1;
        if (N == 0) return 0;
        for (int i = 2; i <= N; i++) {
            int old = ret;
            ret += pre;
            pre = old;
        }
        return ret;
    }
    
}