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);    }}