【广度优先搜索】

#include <stdio.h>
#include <string.h>
#include <queue>
using namespace std;

int sx, sy, ex = 3, ey = 4;
int dir[8][2] = {1, 1, 1, -1, -1, -1, -1, 1, 0, -1, 1, 0, -1, 0, 0, 1};
char x1, y1, x2, y2;

struct Node
{
    int x;
    int y;
    int px;
    int py;
    int d;
    int step;
    int visit;
};

Node Susake[200][200];
int map[200][200], sum;
char path[200][300];

void bfs(int a, int b)
{
    memset(Susake, 0, sizeof(Susake));
    queue <Node> q;
    Node cur;

    Susake[sx][sy].x = a;
    Susake[sx][sy].y = b;
    Susake[sx][sy].px = a;
    Susake[sx][sy].py = b;
    Susake[sx][sy].step = 0;
    Susake[sx][sy].visit = 1;

    q.push(Susake[sx][sy]);

    if(sx == ex && sy == ey)
    {
        printf("%d\n", sum);
        return ;
    }

    while(!q.empty())
    {
        cur = q.front();
        q.pop();
        if(cur.x == ex && cur.y == ey)
        {
            sum = cur.step;
            printf("%d\n", sum);
            int tx = Susake[ex][ey].px, ty = Susake[ex][ey].py;
            int i = 0;
            memset(path, 0, sizeof(path));
            if(tx < ex && ty < ey) strcpy(path[i++], "RD");
            if(tx == ex && ty < ey) strcpy(path[i++], "R");
            if(tx < ex && ty == ey) strcpy(path[i++], "D");
            if(tx == ex && ty > ey) strcpy(path[i++], "L");
            if(tx > ex && ty == ey) strcpy(path[i++], "U");
            if(tx < ex && ty > ey) strcpy(path[i++], "LD");
            if(tx > ex && ty > ey) strcpy(path[i++], "LU");
            if(tx > ex && ty < ey) strcpy(path[i++], "RU");
            //printf("%d %d %d %d\n", tx, ty, ex, ey);
            ex = tx; ey = ty;
            while(tx != sx || ty != sy)
            {
                tx = Susake[tx][ty].px;
                ty = Susake[tx][ty].py;
                //printf("%d %d %d %d\n", tx, ty, ex, ey);
                if(tx < ex && ty < ey) strcpy(path[i++], "RD");
                if(tx == ex && ty < ey) strcpy(path[i++], "R");
                if(tx < ex && ty == ey) strcpy(path[i++], "D");
                if(tx == ex && ty > ey) strcpy(path[i++], "L");
                if(tx > ex && ty == ey) strcpy(path[i++], "U");
                if(tx < ex && ty > ey) strcpy(path[i++], "LD");
                if(tx > ex && ty > ey) strcpy(path[i++], "LU");
                if(tx > ex && ty < ey) strcpy(path[i++], "RU");
                //printf("%d %d %d %d\n", tx, ty, ex, ey);
                ex = tx; ey = ty;
            }
            for(int j = i - 1; j >= 0; j--)
                printf("%s\n", path[j]);
            break;
        }
        for(int i = 0; i < 8; i++)
        {
            int tx = cur.x + dir[i][0];
            int ty = cur.y + dir[i][1];
            if(Susake[tx][ty].visit == 0 && map[tx][ty])
            {
                Susake[tx][ty].x = tx;
                Susake[tx][ty].y = ty;
                Susake[tx][ty].step = cur.step + 1;
                Susake[tx][ty].visit = 1;
                //save PREVIOUS
                Susake[tx][ty].px = cur.x;
                Susake[tx][ty].py = cur.y;
                Susake[tx][ty].d = i;
                q.push(Susake[tx][ty]);
            }
        }
    }
}

int main(int argc, char *argv[])
{
    memset(map, 0, sizeof(map));
    for(int i = 1; i <= 8; i++)
        for(int j = 1; j <= 8; j++) map[i][j] = 1;
    scanf("%c%c%*c%c%c", &x1, &y1, &x2, &y2);
    sum = 0;
    sx = 57 - y1; sy = x1 - 96;
    ex = 57 - y2; ey = x2 - 96;
    //printf("%d %d %d %d\n", sx, sy, ex, ey);
    bfs(sx, sy);
    return 0;
}