C++ class中寫sort二級排序

　　做tc時被這個問題糾結住了，後來問了大牛，可以這樣寫。

 1 class EvenRoute {
 2 public:
 3     struct node {
 4         int x;
 5         int y;
 6         node(int a = 0, int b = 0) : x(a), y(b) {}
 7     };
 8 
 9     static bool cmp(const node& c, const node& d) {   //加static
10         if(c.x == d.x)    return c.y < d.y;
11         return c.x < d.x;
12     }
13 
14     string isItPossible(vector <int> x, vector <int> y, int w) {
15         struct node a[1000];
16         int n = 1000;
17         //....
18         sort(a, a + n, cmp);
19         return "CAN";
20     }
21 };