题目

104. 建造最大岛屿 (kamacoder.com)

代码（ACM 首刷看解析）：

#include<iostream>
#include<vector>
#include<unordered_map>
#include<unordered_set>
using namespace std;

int dir[4][2] = {1,0,-1,0,0,1,0,-1};
int count;
int n, m;
void dfs(vector<vector<int>>&grid, int mark, int row, int col) {
    grid[row][col] = mark;
    count++;
    
    for (int i = 0; i < 4; i++) {
        int newRow = row + dir[i][0];
        int newCol = col + dir[i][1];
        if (newRow < 0 || newRow >= n || newCol < 0 || newCol >= m) continue;
        if (grid[newRow][newCol] != 1) continue;
        dfs(grid, mark, newRow, newCol);
    }
}

void print(vector<vector<int>>& arr, int n, int m) {
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cout << arr[i][j] <<" ";
        }
        cout << endl;
    }
}

int main() {
    cin >> n >> m;
    vector<vector<int>> grid(n, vector<int>(m));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> grid[i][j];
        }
    }
    
    // print(grid, n, m);
    // cout<<endl;
    // 先遍历一遍，获得所有岛屿的最大值
    int mark = 2;
    count = 0;
    unordered_map<int,int> myMap;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (grid[i][j] == 1) {
                dfs(grid, mark, i, j);
                myMap[mark] = count;
                mark++;
                count = 0;
            }
        }
    }
    
    //print(grid, n, m);
    
    int maxResult = 0;
    unordered_set<int> us;
    bool AllisGrid = true;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (grid[i][j] == 0) {
                AllisGrid = false;
                int tmp = 0;
                for (int k = 0; k < 4; k++) {
                    int newI = i + dir[k][0];
                    int newJ = j + dir[k][1];
                    if (newI < 0 || newI >= n || newJ < 0 || newJ >= m) continue;
                    if (us.find(grid[newI][newJ]) == us.end()) {
                        tmp += myMap[grid[newI][newJ]];
                        us.insert(grid[newI][newJ]);
                    }
                }
                maxResult = max(tmp + 1, maxResult);
                us.clear();
                
                // cout << endl;
                // cout << "grid[" << i << "][" << j << "]:" << tmp << endl;
            } 
        }
    }
    if (AllisGrid) cout << n * m << endl;
    else cout << maxResult << endl;
    
    return 0;
}