2.2 dfs 矩阵遍历

力扣 200 岛屿数量

public class numIslands_1 {
    private int res;
    public int numIslands(char[][] grid) {
        res = 0;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                if(grid[i][j] == '1'){
                    dfs(grid, i , j);
                    res ++;
                }
            }
        }
        return res;
    }


    private void dfs(char[][] grid, int i, int j) {
        if(i >= grid.length || j >= grid[0].length || i < 0 || j < 0){
            return;
        }
        if(grid[i][j] != '1'){
            return ;
        }
        grid[i][j] = '2';
        dfs(grid, i-1,j);
        dfs(grid, i+1,j);
        dfs(grid, i,j-1);
        dfs(grid, i,j+1);
    }
}

力扣 695 岛屿的最大面积

• 进阶版dfs 矩阵遍历

class Solution {
    public int maxAreaOfIsland(int[][] grid) {
        int res = 0;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                int cur = dfs(i, j , grid);
                res = Math.max(cur, res);
            }
        }
        return res;
    }
    public int dfs(int i, int j, int[][] nums){
        if(i < 0 || i >= nums.length || j < 0 || j >= nums[0].length){
            return 0;
        }
        if(nums[i][j] != 1){
            return 0;
        }

        nums[i][j] = 2;
        return 1 + dfs(i - 1, j, nums)
            + dfs(i, j - 1, nums)
            + dfs(i + 1, j , nums)
            + dfs(i, j + 1, nums);
    }
}

力扣827 最大人工岛

class Solution {
    public static int largestIsland(int[][] grid) {
        int res = 0, idx = 2;
        HashMap<Integer, Integer> map = new HashMap<>();
        if(grid == null || grid.length == 0)    return 1;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                if(grid[i][j] == 1){
                    int area = dfs(grid, i, j, idx);
                    map.put(idx ++, area);
                    res = Math.max(res, area);
                }
            }
        }
        if(res == 0)    return 1;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                if(grid[i][j] == 0){
                    HashSet<Integer> set = new HashSet<>();
                    getNeighbor(grid, i, j, set);
                    int cur = 1;
                    if(set.size() < 1)  continue;
                    for(Integer setItem : set){
                        cur += map.get(setItem);
                    }
                    res = Math.max(res, cur);
                }
            }
        }
        return res;
    }


    private static void getNeighbor(int[][] grid, int i, int j, HashSet<Integer> set) {
        if(i - 1 >= 0 && grid[i-1][j] != 0){
            set.add(grid[i-1][j]);
        }
        if(j - 1 >= 0 && grid[i][j-1] != 0){
            set.add(grid[i][j-1]);
        }
        if(i + 1 < grid.length && grid[i+1][j] != 0){
            set.add(grid[i+1][j]);
        }
        if(j + 1 < grid[0].length && grid[i][j+1] != 0){
            set.add(grid[i][j+1]);
        }
    }


    private static int dfs(int[][] grid, int i, int j, int idx) {
        if(i < 0 || j < 0 || i >= grid.length || j >= grid[0].length)    return 0;
        if(grid[i][j] == 1) {
            grid[i][j] = idx;
            return 1 + dfs(grid, i - 1, j, idx)
                    + dfs(grid, i + 1, j, idx)
                    + dfs(grid, i, j - 1, idx)
                    + dfs(grid, i, j + 1, idx);
        }else{
            return 0;
        }
    }
}

力扣 463岛屿的周长

class Solution {
    public int islandPerimeter(int[][] grid) {
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                if(grid[i][j] == 1){
                    return dfs(grid, i, j);
                }
            }
        }
        return 0;
    }


    public int dfs(int[][] grid, int i, int j){
        if(i < 0 || j < 0 || i >= grid.length || j >= grid[0].length){
            return 1;
        }
        if(grid[i][j] == 0){
            return 1;
        }
        if(grid[i][j] == 2){
            return 0;
        }
        grid[i][j] = 2;
        return dfs(grid, i-1, j) + 
            dfs(grid, i+1, j) + 
            dfs(grid, i, j-1) + 
            dfs(grid, i, j+1);
    }
}