Given a binary tree, return the *preorder* traversal of its nodes’ values.

For example:

Given binary tree `{1,#,2,3}`

,

1 \ 2 / 3

return `[1,2,3]`

.

**Note:** Recursive solution is trivial, could you do it iteratively?

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# Category: DFS

Binary Tree · Depth First Search · DFS · leetcode · Stack · Traversals · Trees## Binary Tree Preorder Traversal

Binary Tree · Depth First Search · DFS · leetcode · Stack · Traversals · Trees## Binary Tree Inorder Traversal

Depth First Search · DFS · Graphs · leetcode## Number Of Islands

Given a binary tree, return the *preorder* traversal of its nodes’ values.

For example:

Given binary tree `{1,#,2,3}`

,

1 \ 2 / 3

return `[1,2,3]`

.

**Note:** Recursive solution is trivial, could you do it iteratively?

Given a binary tree, return the *inorder* traversal of its nodes’ values.

For example:

Given binary tree `[1,null,2,3]`

,

1 \ 2 / 3

return `[1,3,2]`

.

**Note:** Recursive solution is trivial, could you do it iteratively?

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:**

11110 11010 11000 00000

Answer: 1

**Example 2:**

11000 11000 00100 00011

Answer: 3