547. Number of Provinces 省份数量

There are n cities. Some of them are connected, while some are not. If city a is connected directly with city b, and city b is connected directly with city c, then city a is connected indirectly with city c.

A province is a group of directly or indirectly connected cities and no other cities outside of the group.

You are given an n x n matrix isConnected where isConnected[i][j] = 1 if the ith city and the jth city are directly connected, and isConnected[i][j] = 0 otherwise.

Return the total number of provinces.

 

Example 1:

Input: isConnected = [[1,1,0],[1,1,0],[0,0,1]]
Output: 2

Example 2:

Input: isConnected = [[1,0,0],[0,1,0],[0,0,1]]
Output: 3

和数字岛屿的区别：
这个例子

0,0 0,3
1,1 1,2
2,1 2,2 2,3
3,0 3,2 3,3
0123全都是相通的，所以pyq数量=1。但是岛屿数量是4.

您有一个 NxN 矩阵，但总共只有 N 个朋友。所以一行只loop一次就行了。

别的特点：本题对角元素全为1。

教训：题目改名字了，最好重新搜索一下。一定要小心再小心！

做法是修改一下dfs

public class Solution {

private int n;
private int m;

public int numIslands(char[][] grid) {
    int count = 0;
    n = grid.length;
    if (n == 0) return 0;
    m = grid[0].length;
    for (int i = 0; i < n; i++){
        for (int j = 0; j < m; j++)
            if (grid[i][j] == '1') {
                DFSMarking(grid, i, j);
                ++count;
            }
    }    
    return count;
}

private void DFSMarking(char[][] grid, int i, int j) {
    if (i < 0 || j < 0 || i >= n || j >= m || grid[i][j] != '1') return;
    grid[i][j] = '0';
    
    for (int d = 0; d < n; d++)
        dfs(grid, d, j);

    for (int d = 0; d < m; d++)
        dfs(grid, i, d);
}
}