POJ3669 Meteor Shower [BFS]

Meteor Shower

Time Limit:1000 MS		Memory Limit:65536 K
Total Submissions:9163		Accepted:2594

Description

Bessie hears that an extraordinary meteor shower is coming; reports say that these meteors will crash into earth and destroy anything they hit. anxious for her safety, she vows to find her way to a safe location (one that is never destroyed by a meteor ). she is currently grazing at the origin in the coordinate plane and wants to move to a new, safer location while avoiding being destroyed by meteors along her way.

The reports say thatMMeteors (1 ≤M≤ 50,000) will strike, with meteorIWill striking point (Xi, Yi) (0 ≤Xi≤ 300; 0 ≤Yi≤ 300) at timeTi(0 ≤Ti≤ 1,000). Each meteor destroys the point that it strikes and also the four rectilinearly adjacent lattice points.

Bessie leaves the origin at time 0 and can travel in the first quadrant and parallel to the axes at the rate of one distance unit per second to any of the (often 4) adjacent rectilinear points that are not yet destroyed by a meteor. she cannot be located on a point at any time greater than or equal to the time it is destroyed ).

Determine the minimum time it takes Bessie to get to a safe place.

Input

* Line 1: A single integer:M
* Lines 2 ..M+ 1: LineI+ 1 contains three space-separated integers:Xi, Yi, AndTi

Output

* Line 1: The minimum time it takes Bessie to get to a safe place or-1 if it is impossible.

Sample Input

40 0 22 1 21 1 20 3 5

Sample Output

5

Source

USACO 2008 February Silver

/*** Problem: POJ No. 3669 *** Running time: 94 MS *** Complier: C ++ *** Author: Changmu: given the location and time of the explosion of n bombs, find the minimum time that James ran from (0, 0) to the safe area. ** Problem solving: direct BFS. ** Pitfalls: The data range is not viewed. In fact, the security zone can be out of the given range and may be cracked at the beginning... */# include <stdio. h> # include <string. h >#include <queue> # define maxn 305 # define maxT 1005int map [maxn] [maxn]; const int mov [] [2] = {0, 1, 0, -1,-1, 0, 1, 0,}; int N; struct Node {int x, y, t ;}; void Destory (int x, int y, int t) {if (map [x] [y] =-1 | t <map [x] [y]) map [x] [y] = t; int xa, ya; for (int I = 0; I <4; ++ I) {xa = x + mov [I] [0]; ya = y + mo V [I] [1]; if (xa <0 | ya <0) continue; if (-1 = map [xa] [ya] | t <map [xa] [ya]) map [xa] [ya] = t ;}} void getMap () {memset (map,-1, sizeof (map); int I, j, x, y, t; for (I = 0; I <N; ++ I) {scanf ("% d", & x, & y, & t); Destory (x, y, t );}} bool check (int x, int y, int t) {if (x <0 | y <0) return false; if (map [x] [y] =-1 | map [x] [y]> t) return true; return false;} int BFS () {if (map [0] [0] =-1 ) Return 0; if (! Map [0] [0]) return-1; std: queue <Node> Q; Node u, v; u. x = u. y = u. t = 0; Q. push (u); while (! Q. empty () {u = Q. front (); Q. pop (); for (int I = 0; I <4; ++ I) {v = u; ++ v. t; v. x + = mov [I] [0]; v. y + = mov [I] [1]; if (check (v. x, v. y, v. t) {if (map [v. x] [v. y] =-1) return v. t; map [v. x] [v. y] = 0; Q. push (v) ;}}return-1 ;}int main () {// freopen ("stdin.txt", "r", stdin ); while (scanf ("% d", & N) = 1) {getMap (); printf ("% d \ n", BFS ();} return 0 ;}

POJ3669 Meteor Shower [BFS]