UVA 11882 Biggest Number 深搜 剪枝

#include <iostream>
#include <cstring>
#include <queue>
#include <vector>
#include <algorithm>
using namespace std;

const int MAX = 20;int dx[4] = {-1, 0, 1, 0};
int dy[4] = { 0, 1, 0,-1};
char G[MAX][MAX];
int vis[MAX][MAX];
int vis2[MAX][MAX];
string ans;
int r, c;

void dfs(int i, int j, string sum);        //暴力求解 
int getLen(int i, int j);                //求在 i，j点开始走，最长的路径是多少 
string maxSum(string s1, string s2);    //比较两个数字 

int main(){
    freopen("input.txt", "r", stdin);
    
    while(cin >> r >> c){
        if(r == 0 && c == 0)
            break;
        
        for(int i=1; i<=r; i++){
            for(int j=1; j<=c; j++){
                cin >> G[i][j];
            }
        }
        
        ans = "";
        
        for(int i=1; i<=r; i++){
            for(int j=1; j<=c; j++){
                if(G[i][j] != ‘#‘){
                    memset(vis, 0, sizeof(vis));
                    dfs(i, j, "");
                }
            }
        }
        
        cout << ans << endl;
    }
    
    return 0;
} 

void dfs(int i, int j, string sum){
    if(vis[i][j] == 1)
        return ;
    
    sum = sum + G[i][j];
    memcpy(vis2, vis, sizeof(vis)); 
    int len = getLen(i, j) - 1;
    
    if(len + sum.length() < ans.length()){        //长度不够 
        return ;
    }
    
    if(len + sum.length() == ans.length()){        //长度相同，但不可能比答案更大 
        if(sum < ans.substr(0, sum.length()))
            return ;
    }
    
    if(len == 0){        //更新一个答案 
        ans = maxSum(ans, sum);
        return ;
    }
    
    vis[i][j] = 1;
    
    for(int k=0; k<4; k++){
        int x = i + dx[k], y = j + dy[k];
        if(x < 1 || x > r || y < 1 || y > c || G[x][y] == ‘#‘ || vis[x][y] == 1){
            continue;
        }else{
            dfs(x, y, sum);
        }
    }
    
    vis[i][j] = 0;
    
}

int getLen(int i, int j){
    if(vis2[i][j] == 1)
        return 0;
    vis2[i][j] = 1;
    int sum = 1;
    for(int k=0; k<4; k++){
        int x = i + dx[k];
        int y = j + dy[k];
        if(x < 1 || x > r || y < 1 || y > c || vis2[x][y] || G[x][y] == ‘#‘)
            continue;
        sum += getLen(x, y);
    }
    return sum;
}

string maxSum(string s1, string s2){
    if(s1.length() == s2.length())
        return max(s1, s2);
    else{
        if(s1.length() > s2.length())
            return s1;
        return s2;
    }
}