# Posts tagged as “DFS”

Problem:

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

Example 2:

# Idea: DFS

Use DFS to find a connected component (an island) and mark all the nodes to 0.

Time complexity: O(mn)

Space complexity: O(mn)

# Related Problems

Problem:

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Example 1:

Time Complexity:

O(n)

Space Complexity:

O(h)

Solution 1:

BFS

Solution 2:

DFS

Related Problems:

Given a 2D board and a word, find if the word exists in the grid.

The word can be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once.

For example,
Given board =

word = "ABCCED", -> returns true,
word = "SEE", -> returns true,
word = "ABCB", -> returns false.

Idea:
Search, depth first search
Solution:

## Python

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

Solution 1: BFS O(n)

Solution 2: DFS O(n)