bfs 广度优先遍历 $O(nm)$

C++ 代码

//o(nm) bfs
#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 21;
int w, h;
char g[N][N];
typedef pair<int, int> PII;

int bfs(int x, int y){
    queue<PII> q;
    q.push({x, y});
    g[x][y] = '#';

    int dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};

    int res = 0;
    while (q.size()){
        auto t = q.front();
        q.pop();
        res ++;

        for (int i = 0; i < 4; i ++ ){
            int a = t.first + dx[i], b = t.second + dy[i];
            if (a < 0 || a >= h || b < 0 || b >= w || g[a][b] != '.') continue;
            g[a][b] = '#';
            q.push({a, b});
        }
    }
    return res;

}

int main(){
    while (cin >> w >> h, w || h){
        for (int i = 0; i < h; i ++ ) cin >> g[i];
        int x, y;
        for (int i = 0; i < h; i ++ )
            for (int j = 0; j < w; j ++ ){
                if (g[i][j] == '@'){
                    x = i, y = j;
                }
            }

        cout << bfs(x, y) << endl;
        // cout << dfs(x, y) << endl;
    }
    return 0;

}

dfs 深度优先遍历

C++ 代码

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 21;
int w, h;
char g[N][N];
typedef pair<int, int> PII;
int dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};

int dfs(int x, int y){
    g[x][y] = '#';
    int res = 1;

    for (int i = 0; i < 4; i ++ ){
        int a = x + dx[i], b = y + dy[i];
        if (a >= 0 && a < h && b >= 0 && b < w && g[a][b] == '.'){
            res += dfs(a, b);
        }
    }
    return res;
}

int main(){
    while (cin >> w >> h, w || h){
        for (int i = 0; i < h; i ++ ) cin >> g[i];
        int x, y;
        for (int i = 0; i < h; i ++ )
            for (int j = 0; j < w; j ++ ){
                if (g[i][j] == '@'){
                    x = i, y = j;
                }
            }
        cout << dfs(x, y) << endl;
    }
    return 0;

}