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# Fibonacci

ID: 366; naive; 斐波纳契数列

## "Solution 1" (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
if (n <= 1) return 0;
if (n == 2) return 1;
return fibonacci(n - 1) + fibonacci(n - 2);
}
}

### Notes

• Classical recursion example
• This actually will not pass the test on LintCode (test a relatively large Fibonacci number say the 50th).
• Time complexity: O(2^n), extremely bad exponential runtime.
• Space complexity: O(1), excluding stack space (O(n)).

## Solution 2 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
int[] memo = new int[n + 1];
return fibHelper(n, memo);
}
private int fibHelper(int n, int[] memo) {
if (n <= 1) return 0;
if (n == 2) return 1;
if (memo[n] > 0) return memo[n];
memo[n] = fibHelper(n - 1, memo) + fibHelper(n - 2, memo);
return memo[n];
}
}

### Notes

• This solution uses an array to store the Fibonacci values by the concept of memoization.
• Time complexity: O(n)
• Space complexity: O(n)

## Solution 3 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
int n1 = 0, n2 = 1, sum = 0;
for (int i = 1; i < n; i++) {
sum = n1 + n2;
n1 = n2;
n2 = sum;
}
return n1;
}
}

### Notes

• This solution also uses the idea of memoization, but it uses a constant number of variables to only store the most important information (current 3 numbers).
• n1, n2, sum are the current two numbers and the previous sum of the two numbers.
• Time complexity: O(n)
• Space complexity: O(1)

## Solution 4 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
int[] fib = new int[]{0, 1, 1};
for (int i = 3; i < n; i++) {
fib[i % 3] = fib[(i-1) % 3] + fib[(i-2) % 3];
}
return fib[(n-1) % 3];
}
}

### Notes

• This solution is similar to Solution 2 by using a length-3 array to store the 3 important numbers.
• It is also somewhat different because the third element of the array is indeed the current sum of the first and second elements.
• Time complexity: O(n)
• Space complexity: O(1)

## Solution 5 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
int[][] fib = new int[][] { { 1, 1 }, { 1, 0 } };
if (n <= 1) return 0;
power(fib, n);
return fib[0][0];
}
private void power(int[][] fib, int n) {
int[][] fibCopy = new int[][] { { 1, 1 }, { 1, 0 } };
for (int i = 0; i < n - 3; i++) {
matrixMul(fib, fibCopy);
}
}
private void matrixMul(int[][] fib, int[][] fibCopy) {
int x = fib[0][0] * fibCopy[0][0] + fib[0][1] * fibCopy[1][0];
int y = fib[0][0] * fibCopy[0][1] + fib[0][1] * fibCopy[1][1];
int z = fib[1][0] * fibCopy[0][0] + fib[1][1] * fibCopy[1][0];
int w = fib[1][0] * fibCopy[0][1] + fib[1][1] * fibCopy[1][1];
fib[0][0] = x;
fib[0][1] = y;
fib[1][0] = z;
fib[1][1] = w;
}
}

### Notes

$\begin{pmatrix} 1 & 1 \\ 1 & 0 \end{pmatrix}^n = \begin{pmatrix} F_{n+1} & F_n \\ F_n & F_{n-1} \end{pmatrix}$
• By the above mathematical rule of Fibonacci numbers, we have this solution.
• Note that the loop does not run for n = 1, 2, 3 nor any non-positive numbers. So it starts with n = 4 in the code, but n is actually 2 in the formula and the top-left element would be F3, which is the correct result for the input n = 4 in the code.
• Time complexity: O(n)
• Space complexity: O(1)

## Solution 6 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
int[][] fib = new int[][] { { 1, 1 }, { 1, 0 } };
if (n <= 1) return 0;
power(fib, n - 2);
return fib[0][0];
}
private void power(int[][] fib, int n) {
if (n == 0 || n == 1) return;
int[][] fibCopy = new int[][] { { 1, 1 }, { 1, 0 } };
power(fib, n / 2);
matrixMul(fib, fib);
if (n % 2 != 0) matrixMul(fib, fibCopy);
}
private void matrixMul(int[][] fib, int[][] fibCopy) {
int x = fib[0][0] * fibCopy[0][0] + fib[0][1] * fibCopy[1][0];
int y = fib[0][0] * fibCopy[0][1] + fib[0][1] * fibCopy[1][1];
int z = fib[1][0] * fibCopy[0][0] + fib[1][1] * fibCopy[1][0];
int w = fib[1][0] * fibCopy[0][1] + fib[1][1] * fibCopy[1][1];
fib[0][0] = x;
fib[0][1] = y;
fib[1][0] = z;
fib[1][1] = w;
}
}

### Notes

• Using the same idea in Solution 5, we optimize the power function by using divide and conquer. This idea is also illustrated in this Pow(x, n) problem.
• Time complexity: O(log n)
• Space complexity: O(1)
This solution can also be extended by deriving a recursive relation from the matrix multiplication. The formula/relation is:
• When n is even and k = n / 2,
$F(n) = [2 \times F(k -1) + F(k)] \times F(k)$
• When n is even and k = (n+1) / 2,
$F(n) = F(k) \times F(k) + F(k-1) \times F(k-1)$
• The derivation is shown here:
• This extension would have the same time and space complexity as done in code above. The reason is that this is still essentially divide and conquer where k is half of n each time.

## Solution 7 (Java)

public class Solution {
/**
* @param n: an integer
* @return: an ineger f(n)
*/
public int fibonacci(int n) {
double phi = (1 + Math.sqrt(5)) / 2;
return (int) Math.round(Math.pow(phi, --n) / Math.sqrt(5));
}
}

### Notes

$F(n) = \frac{1}{\sqrt{5}}[(\frac{1+\sqrt{5}}{2})^n - (\frac{1-\sqrt{5}}{2})^n]$
• This solution uses the mathematical formula for calculation (not the recursive relation).
• Note that n-- because the problem is 1-indexed.
• Time complexity: O(log n) with the assumption that Math.pow takes log n time.
• Space complexity: O(1).