hihocoder1696 折线中点（几何）

https://hihocoder.com/problemset/problem/1696

求折线中点，一开始想成先求横坐标中点了，肯定是错的。

一定要从线段长度求中点，然后中点公式推了好久。。

 #include<iostream>
 #include<cstdio>
 #include<queue>
 #include<cstring>
 #include<algorithm>
 #include<cmath>
 #include<map>
 #define lson l, m, rt<<1
 #define rson m+1, r, rt<<1|1
 #define IO ios::sync_with_stdio(false);cin.tie(0);
 #define INF 0x3f3f3f3f
 typedef long long ll;
 using namespace std;
 int n;
 typedef struct{
     int x, y;
 }Node;
 Node node[];
 double a[];
 double dist(int a, int b)
 {
     return sqrt(a*a+b*b);
 }
 int main()
 {
     double sum = ;
     cin >> n;
     for(int i = ; i < n; i++){
         cin >> node[i].x >> node[i].y;
     }
     for(int i = ; i < n-; i++){
         sum += dist(node[i+].x-node[i].x, node[i+].y-node[i].y);
         a[i] = sum;//存放到点i+1为止的折线长度
     }
     sum /= ;
     int i;
     for(i = ; i < n-; i++){
         if(sum < a[i]||sum-a[i]<1e-){//小于或等于
             break;
         }
     }
     sum -= a[i-];
     double tx = node[i+].x-node[i].x;
     double ty = node[i+].y-node[i].y;
     double midx = sum/dist(node[i+].x-node[i].x, node[i+].y-node[i].y)*tx+node[i].x;
     double midy = sum/dist(node[i+].x-node[i].x, node[i+].y-node[i].y)*ty+node[i].y;
     printf("%.1lf %.1lf\n", midx, midy);
     return ;
 }