题目：

题解：

type matrix [2][2]int

func multiply(a, b matrix) (c matrix) {
    for i := 0; i < 2; i++ {
        for j := 0; j < 2; j++ {
            c[i][j] = a[i][0]*b[0][j] + a[i][1]*b[1][j]
        }
    }
    return
}

func pow(a matrix, n int) matrix {
    ret := matrix{{1, 0}, {0, 1}}
    for ; n > 0; n >>= 1 {
        if n&1 == 1 {
            ret = multiply(ret, a)
        }
        a = multiply(a, a)
    }
    return ret
}

func fib(n int) int {
    if n < 2 {
        return n
    }
    res := pow(matrix{{1, 1}, {1, 0}}, n-1)
    return res[0][0]
}