斐波那契数列

// ====================方法1：递归====================long long Fibonacci_Solution1(unsigned int n){    if(n <= 0)        return 0;    if(n == 1)        return 1;    return Fibonacci_Solution1(n - 1) + Fibonacci_Solution1(n - 2);}

// ====================方法2：循环====================long long Fibonacci_Solution2(unsigned n){    int result[2] = {0, 1};    if(n < 2)        return result[n];    long long  fibNMinusOne = 1;    long long  fibNMinusTwo = 0;    long long  fibN = 0;    for(unsigned int i = 2; i <= n; ++ i)    {        fibN = fibNMinusOne + fibNMinusTwo;        fibNMinusTwo = fibNMinusOne;        fibNMinusOne = fibN;    }     return fibN;}

// ====================方法3：基于矩阵乘法====================#include <cassert>struct Matrix2By2{    Matrix2By2    (        long long m00 = 0,         long long m01 = 0,         long long m10 = 0,         long long m11 = 0    )    :m_00(m00), m_01(m01), m_10(m10), m_11(m11)     {    }    long long m_00;    long long m_01;    long long m_10;    long long m_11;};Matrix2By2 MatrixMultiply(    const Matrix2By2& matrix1,     const Matrix2By2& matrix2){    return Matrix2By2(        matrix1.m_00 * matrix2.m_00 + matrix1.m_01 * matrix2.m_10,        matrix1.m_00 * matrix2.m_01 + matrix1.m_01 * matrix2.m_11,        matrix1.m_10 * matrix2.m_00 + matrix1.m_11 * matrix2.m_10,        matrix1.m_10 * matrix2.m_01 + matrix1.m_11 * matrix2.m_11);}Matrix2By2 MatrixPower(unsigned int n){    assert(n > 0);    Matrix2By2 matrix;    if(n == 1)    {        matrix = Matrix2By2(1, 1, 1, 0);    }    else if(n % 2 == 0)    {        matrix = MatrixPower(n / 2);        matrix = MatrixMultiply(matrix, matrix);    }    else if(n % 2 == 1)    {        matrix = MatrixPower((n - 1) / 2);        matrix = MatrixMultiply(matrix, matrix);        matrix = MatrixMultiply(matrix, Matrix2By2(1, 1, 1, 0));    }    return matrix;}long long Fibonacci_Solution3(unsigned int n){    int result[2] = {0, 1};    if(n < 2)        return result[n];    Matrix2By2 PowerNMinus2 = MatrixPower(n - 1);    return PowerNMinus2.m_00;}