* 用改进的欧拉方法求解初值问题，其中一阶微分方程未y =f(x,y)

* 初始条件为x=x[0]时,y＝y[0].

* 输入: f--函数f(x,y)的指针

* x--自变量离散值数组(其中x[0]为初始条件)

* y--对应于自变量离散值的函数值数组(其中y[0]为初始条件)

* h--计算步长

* n--步数

* 输出: x为说求解的自变量离散值数组

* y为所求解对应于自变量离散值的函数值数组

-* Improved Euler method to solve initial value problems, not an order differential equation y = f (x, y) * initial conditions for x = x [0], y = y [0]. * Input : f -- function f (x, y) * x pointer -- from the array variable discrete values (x [0] for the initial conditions) * y -- corresponding to the variable discrete value of the function of the array (y [0] for the initial conditions) * h -- calculated step * n -- steps * output : Solving for x that the variable array * discrete values by solving y variables corresponding to the value of the discrete function arrays