Fibonacci(lintcode 366)

Description

Find the Nth number in Fibonacci sequence.

A Fibonacci sequence is defined as follow:

• The first two numbers are 0 and 1.
• The i th number is the sum of i-1 th number and i-2 th number.

The first ten numbers in Fibonacci sequence is:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ...

Notice:
The Nth fibonacci number won't exceed the max value of signed 32-bit integer in the test cases.

Example

Given 1, return 0

Given 2, return 1

Given 10, return 34

Interface

class Solution {
    /**
     * @param n: an integer
     * @return an integer f(n)
     */
    public int fibonacci(int n) {
        // write your code here
    }
}

Idea

Just build the loop according to the definition of Fibonacci sequence.

Solution

class Solution {
    /**
     * @param n: an integer
     * @return an integer f(n)
     */
    public int fibonacci(int n) {
        // write your code here
        if (n == 1) {
            return 0;
        } else if (n == 2) {
            return 1;
        }

        int preSum = 0;
        int sum = 1;
        for (int i = 2; i < n; ++i) {
            sum += preSum;
            preSum = sum - preSum;
        }
        return sum;
    }
}