C++实现多项式拟合计算, y=a+bx+cx^2+dx^3,给出n对x,y值后,得出abcd值,以后给出x的值,自动计算y的值-C++ implementation polynomial fitting calculation, y = a+ bx+ cx ^ 2+ dx ^ 3, n is given on the x, y values, results in abcd value that is given after the value of x, y values ​