广度优先搜索（Breadth First Search,BFS）

广度优先搜索

思想：以一个未被访问的结点作为起始结点，访问其所有相邻结点，然后对每一个相邻的结点，再访问它们相邻的未被访问的结点。

实现：使用队列从起始结点到子结点扩展，在遍历图中相当于前序遍历

Sample In

5 4

0 0 1 0

0 0 0 0

0 0 1 0

0 1 0 0

0 0 0 1

1 1 4 3

Sample Out

7

//C代码
#include <cstdio>
#include <cstring>
#include <iostream>

using namespace std;
struct note{
int x;
int y;
int s;//步数
};

int main(){
struct note que[2501];//队列
int head,tail;//队列头尾指针
int i,j,k,n,m,startx,starty,p,q,tx,ty,flag;
int next[4][2]={{0,1},{1,0},{0,-1},{-1,0}};
int a[51][51]={0};
int book[51][51]={0};
scanf("%d %d",&n,&m);
for(i=1;i<=n;i++)
for(j=1;j<=m;j++)
scanf("%d",&a[i][j]);
scanf("%d %d %d %d",&startx,&starty,&p,&q);
//队列初始化
head =1;
tail =1;
que[tail].x=startx;
que[tail].y=starty;
que[tail].s=0;
tail++;
book[startx][starty]=1;
flag=0;
while(head<tail){
for(k=0;k<=3;k++){
tx=que[head].x+next[k][0];
ty=que[head].y+next[k][1];
//判断是否越界
if(tx<1||tx>n||ty<1||ty>m)
continue;
//判断是否是障碍物或已走过
if(a[tx][ty]==0&&book[tx][ty]==0){
//加入队列
book[tx][ty] =1;
que[tail].x=tx;
que[tail].y=ty;
que[tail].s=que[head].s+1;
tail++;
}
if(tx==p&&ty==q){
flag=1;
break;
}
}
if(flag==1) break;
head++;
}
printf("%d",que[tail-1].s);
return 0;
}

//Java代码
import java.util.Scanner;

public class Main {
static int m,n;
static int endX,endY;
static int minStep = 2147483647;
static int [][]next = {{0,1},{1,0},{0,-1},{-1,0}};//顺时针右下左上
static int [][]a = new int[51][51] ;
static int [][]book = new int[51][51];
//模拟队列
static int[]x = new int[51] ;
static int[]y = new int[51] ;
static int[]s = new int[51] ;
public static int bfs(int startx,int starty,int endx,int endy){
//初始化
int head =1,tail =1;
x[tail] = startx;
y[tail] = starty;
s[tail] = 0;
book[head][tail] = 1;
tail++;
int flag = 0;
while(head<tail){
for(int k=0;k<=3;k++){
int tx = x[head] + next[k][0];
int ty = y[head] + next[k][1];
//判断是否越界
if(tx<1||tx>n||ty<1||ty>m) continue;
//判断是否走过
if(book[tx][ty] == 0 && a[tx][ty] == 0){
x[tail] = tx;
y[tail] = ty;
s[tail] = s[head]+1;
book[tx][ty] = 1;
tail++;
}
if(tx==endx && ty==endy){
flag = 1;
break;
}
}
if(flag == 1) break;
head++;
}
return s[tail-1];
}
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
n = in.nextInt();
m = in.nextInt();
//地图读入
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
a[i][j] = in.nextInt();
}
}
//读入起点终点
int startX = in.nextInt();
int startY = in.nextInt();
endX = in.nextInt();
endY = in.nextInt();
book[startX][startY] = 1;
minStep = bfs(startX,startY,endX,endY);
System.out.println("最短路径长度为："+minStep);
}
}