【Usaco 2007 Dec silver】穿越泥地 (Standard IO)

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题意：

FJ的屋子在平面坐标(0, 0)的位置，贝茜所在的牛棚则位于坐标(X,Y) (-500 <= X <= 500; -500 <= Y <= 500)处。当然咯，FJ也看到了地上的所有N(1 <= N <= 10,000)个泥塘，第i个泥塘的坐标为(A_i, B_i) (-500 <= A_i <= 500；-500 <= B_i <= 500)。每个泥塘都只占据了它所在的那个格子。不能走到池塘，而且只能平行x，y轴走。

思路：

bfs，加上记忆化

程序：

 type        Point=record                x,y,w:longint;        end;const        maxn=500;        dx:array [1..4] of longint=(1,-1,0,0);        dy:array [1..4] of longint=(0,0,1,-1);var        a:array [-maxn-2..maxn+1,-maxn-2..maxn+2] of boolean;        x,y,x1,y1,n,i,j:longint;        state:array [1..1000000] of Point;function cheak(x,y:longint):boolean;begin        if a[x,y] then exit(false);        if (x<-500) or (x>500) or (y<-500) or (y>500) then exit(false);        exit(true);end;procedure bfs;var        y1,x1,i,j,head,tail:longint;begin        head:=0; tail:=1;        state[1].x:=0; state[1].y:=0;        state[1].w:=0;        repeat                inc(head);                for i:=1 to 4 do                begin                        x1:=state[head].x+dx[i];                        y1:=state[head].y+dy[i];                        if cheak(x1,y1) then                        begin                                inc(tail);                                a[x1,y1]:=true;                                state[tail].x:=x1;                                state[tail].y:=y1;                                state[tail].w:=state[head].w+1;                        end;                        if (x1=x) and (y1=y) then                        begin                                writeln(state[tail].w);                                halt;                        end;                end;        until head=tail;end;begin        readln(x,y,n);        for i:=1 to n do        begin                readln(x1,y1);                a[x1,y1]:=true;        end;        bfs;        writeln(-1);end.

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