BFS & DFS

Sat, 21 Mar 2026 15:00:00 +0800

DFS

78.Subset

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class solution{
 public List<List<Integer>> subsets(int[] nums){
 List<List<Integer>> res=new ArrayList<>();
 backtrack(nums,0,new ArrayList<>(),res);
 return res;
 }

 private void backtrack(int[] nums,int start,List<Integer> path,List<List<Integer>> res){
 res.add(new ArrayList<>(path));

 for(int i=path;i<nums.length;i++){
 path.add(nums[i]);
 backtrack(nums,i+1,path,res);
 path.remove(path.size()-1);
 }
 }
}

BFS

LeetCode

733.图像渲染

Method 1: DFS

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class Solution{
 int[] dx={1,0,0,1};
 int[] dy={0,1,-1,0};
 public int[][] floodFill(int[][] image,int sr,int sc,int color){
 int currColor=image[sr][sc];
 if(curColor==color){
 return image;
 }
 int n=image.length,m=image[0].length;
 Queue<int[]> queue=new LinkedList<int[]>();
 queue.offer(new int[]{sr,sc});
 image[sr][sc]=color;
 while(!queue.isEmpty()){
 int[] cell=queue.poll();
 int x=cell[0],y=cell[1];
 for(int i=0;i<4;i++){
 int mx=x+dx[i],my=y+dy[i];
 if(mx>=0&&my>=0&&mx<=n&&my<=m&&image[mx][my]==curColor){
 queue.offer(new int[]{mx,my});
 image[mx][my]=color;
 }
 }
 }
 return image;
 }
}

Method 2: BFS

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class Solution{
 int[] dx={1,0,0,-1};
 int[] dy={0,1,-1,0};
 public int[][] floodFill(int[][] image,int sr,int sc,int color){
 int curColor=image[sr][sc];
 if(curColor!=color){
 dfs(image,sr,sc,curColor,color);
 }
 return image;
 }
 public void dfs(int[][] image,int x,int y,int curColor,int color){
 if(image[x][y]==curColor){
 image[x][y]=color;
 for(int i=0;i<4;i++){
 int mx=x+dx[i],my=y+dy[i];
 if(mx>=0&&mx<=image.length&&my>=0&&my<=image[0].length){
 dfs(image,mx,my,curColor,color);
 }
 }
 }
 }
}

463.岛屿的周长

Method 1: DFS

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class Solution{
 static int[] dx={0,1,0,-1};
 static int[] dy={1,0,-1,0};
 public int islandPerimeter(int[][] grid){
 int n=grid.length,int m=grid[0].length;
 int ans=0;
 for(int i=0;i<n;i++){
 for(int j=0;j<m;j++){
 if(grid[i][j]==1){
 ans+=dfs(i,j,grid,n,m);
 }
 }
 }
 return ans;
 }
 public int dfs(int x,int y,int[][] grid,int n,int m){
 if(x<0||x>=n||y<0||y>m||grid[x][y]==0){
 return 1;
 }
 if(grid[x][y]==2){
 return 0;
 }
 grid[x][y]=2;
 int res=0;
 for(int i=0;i<4;i++){
 int tx=x+dx[i];
 int ty=y+dy[i];
 res+=dfs(tx,ty,grid,n,m);
 }
 return res;
 }
}

OD

小华地图寻宝

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public class Main{
 static int[] digitSum;
 static void calcDigitSum(int maxVal){
 digitSum=new int[maxVal];
 for(int i=0;i<maxVal;i++){
 int num=i,tmp=0;
 while(num>0){
 tmp+=num%10;
 num/=10;
 }
 digitSum[i]=tmp;
 }
 }
 public static void main(String[] args){
 Scanner scanner=new Scanner(System.in);
 int m=scanner.nextInt(),n=scanner.nextInt(),k=scanner.nextInt();
 scanner.close();
 if(m==0||n==0){
 System.out.println(0);
 return;
 }
 calcDisitSum(Math.max(m,n));
 int[][] direct={{-1,0},{1,0},{0,-1},{0,1}};
 Set<Integer> visited=new HashSet<>();
 Queue<Integer> queue=new LinkedList<>();
 queue.add(0);
 visited.add(0);
 int res=1;
 while(!queue.isEmpty()){
 int num=queue.poll();
 int x=num/n,y=num%n;
 for(int[] d:direct){
 int newX=x+d[0],newY=y+d[1];
 int newNum=newX*newY;
 if(newX<0||newY<0||newX>=m||newY>=n||visited.contains(newNum)){
 continue;
 }
 if(digitSum[newX]+digitSum[newY]>k){
 continue;
 }
 res++;
 queue.add(newNum);
 visited.add(newNum);
 }
 }
 System.out.println(res);
 }
}

Algorithm on Sunjianlin's Blog

BFS & DFS

DFS

78.Subset

BFS

LeetCode

733.图像渲染

463.岛屿的周长

OD

小华地图寻宝