acwing算法提高之搜索--多源BFS与双端队列BFS

1 专题说明

本专题用来计算使用多源BFS和双端队列BFS求解的题目。

2 训练

题目1：173矩阵距离

C++代码如下，

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

using namespace std;

const int N = 1010;
int g[N][N];
int d[N][N];
int n, m;

int main() {
    queue<pair<int,int>> q;
    memset(d, -1, sizeof d);
    
    cin >> n >> m;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            char c;
            cin >> c;
            g[i][j] = c - '0';
            if (g[i][j]) {
                q.push(make_pair(i,j));
                d[i][j] = 0;
            }
        }
    }
    
    int dirs[4][2] = {{-1,0}, {1,0}, {0,-1}, {0,1}};
    
    while (!q.empty()) {
        auto t = q.front();
        q.pop();
        
        //t下一步可以走到哪里
        for (int k = 0; k < 4; ++k) {
            int x = t.first + dirs[k][0];
            int y = t.second + dirs[k][1];
            
            if (x < 1 || x > n || y < 1 || y > m) continue;
            if (d[x][y] != -1) continue; 
            
            q.push(make_pair(x,y));
            d[x][y] = d[t.first][t.second] + 1;
        }
    }
    
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            cout << d[i][j] << " ";
        }
        cout << endl;
    }
    
    return 0;
}
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题目2：