int n = 3，m = 8；
1. n = n + m;
    m = n - m;//m = n
    n = n - m;//n = m
   如果n和m的值非常大，两值之和容易超出int的范围
2. n = n ^ m;
    m = n ^ m;//(n ^ m)^m，m = n
    n =  n ^ m;// n ^ m^n，n = m

int n = 3，m = 8；
1. n = n + m;
    m = n - m;//m = n
    n = n - m;//n = m
   如果n和m的值非常大，两值之和容易超出int的范围
2. n = n ^ m;
    m = n ^ m;//(n ^ m)^m，m = n
    n =  n ^ m;// n ^ m^n，n = m