HDU1010 Tempter of the Bone(回溯 + 剪枝)

#include <cstdio>
#include <iostream>
#include <cstring>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
using namespace std;

const int MAXN = 11;
char Gra[MAXN][MAXN];
int stepX[] = {0, 0, 1, -1};
int stepY[] = {-1, 1, 0, 0};
int N, M, T;
int cnt;
int beginX, beginY, endX, endY;
int OK;

int check(int x, int y)
{
    if(Gra[x][y] == ‘O‘)  return 0;   // 走到了已经走过的路
    if(Gra[x][y] == ‘*‘)  return 0;   //走出了边界
    if(Gra[x][y] == ‘X‘)  return 0;    //走到了墙
    if(cnt > T)           return 0;  //在此时走的步数已经大于总步数则
                          return 1;
}

void backtrack(int x, int y)
{
    if(x == endX && y == endY ) //到达终点
    {
       if(cnt == T)          //找到了答案
           OK = 1;
    }
    else
    {
        for(int i = 0; i < 4; i++)  //在这点一共有四种选择（状态）
        {
            int tx =  x + stepX[i];
            int ty =  y + stepY[i];
            if(check(tx, ty))    //检查所选择的状态是否合理
            {
                ++cnt;              //记录走过的步数
                Gra[tx][ty] = ‘O‘;   //标记走过的地方
                if(OK) return;    //尤为重要的剪枝 ，如果找到答案，就不用继续递归,
                backtrack(tx, ty);
                Gra[tx][ty] = ‘.‘;  //恢复现场
                --cnt;

            }
        }
    }
}

int main()
{
    //freopen("in.txt","r", stdin);
    while(~scanf("%d%d%d",&N, &M, &T) && N)
    {
        getchar();
        beginX = beginY =  endX = endY = 0;
        memset(Gra, ‘*‘, sizeof(Gra));
        for(int i = 1; i <= N; i++)
        {
            for(int j = 1; j <= M; j++)
            {
                scanf("%c",&Gra[i][j]);
                if(Gra[i][j] == ‘S‘)
                    beginX = i, beginY = j;
                if(Gra[i][j] == ‘D‘)
                    endX = i, endY = j;
            }
            getchar();
        }
        OK = 0;
        if( (T > M * N) || ( T < (abs(beginX - endX) + abs(beginY - endY)) ) || ( (beginX + beginY + endX + endY + T) & 1 ))//剪枝二、三
        {
            printf("NO\n"); continue;
        }
        cnt = 0;
        Gra[beginX][beginY] = ‘O‘;
        backtrack(beginX, beginY);
        if(OK) printf("YES\n");
        else printf("NO\n");
    }
    return 0;
}