pku 1925 Spiderman(换个循环嵌套顺序)

我是用DP做的。
一开始把坐标放在最外层循环，然后枚举在该坐标之后的所有building，得到下一个坐标点。TLE。
后来换成把building放在外层循环，然后根据建筑物高度枚举可选的坐标点范围，ac。囧。

#include "math.h"
#include <iostream>
using namespace std;
const int maxn=1000000;
double start_height;
int n,min_swing,end_x;
int building[5005][2];
int swing[maxn];
#define MinN(x,y) ((x)<(y)?(x):(y))
int Max_length(double x,double y)//绳子长度小于等于建筑物高度
{
	return sqrt(2*y*start_height-start_height*start_height);
}
void DP()
{
	min_swing=INT_MAX;
	memset(swing,-1,sizeof(swing));
	swing[building[0][0]]=0;
	for(int i=1;i<n;i++)
	{
		int last_position=building[i][0]-Max_length(building[i][0],building[i][1]);
		for(int j=building[i][0]-1;j>=last_position&&j>=building[0][0];j--)
		{
			if(swing[j]==-1||swing[j]+1>=min_swing) continue;
			int next_x=building[i][0]*2-j;
			if(next_x>=end_x) 
			{
				min_swing=MinN(swing[j]+1,min_swing);
				continue;
			}
			if(swing[next_x]==-1) swing[next_x]=swing[j]+1;
			else swing[next_x]=MinN(swing[next_x],swing[j]+1);
		}
	}
}
int main()
{
	int t;
	scanf("%d",&t);
	while(t--)
	{
		scanf("%d",&n);
		min_swing=maxn;
		for(int i=0;i<n;i++)
		{
			scanf("%d%d",&building[i][0],&building[i][1]);
		}
		start_height=building[0][1];
		end_x=building[n-1][0];
		DP();
		if(min_swing==INT_MAX) printf("-1/n");
		else printf("%d/n",min_swing);
	}
	return 0;
}