Reference: LeetCode

Difficulty: Easy

## Problem

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

**Note:** A leaf is a node with no children.

**Example:**

Given binary tree `[3,9,20,null,null,15,7]`

:

1 | 3 |

The return depth should be `3`

.

## Analysis

First, letâ€™s be clear about the definition of `height`

and `depth`

of a tree or a node.

According to CS 61B, EPI p129, and a post on Stack Overflow:

- The
`height`

of a node is the maximum number of`edges / links`

to any leaf.- The
`height`

of a tree is the height of the root node.

- The
- The
`depth`

of a node is the number of`edges / links`

to the root.- The
`maximum depth`

of a tree is`equal`

to the`height`

of the tree.

- The

**However**, according to the problem description, the maximum depth in this problem equals to the maximum number of `nodes`

to any leaf. In other words, it is one plus our formal definition of `height`

/ `maximum depth`

of a tree.

**Methods:**

- Recursion
- DFS (postorder) traverse the tree.
**Time:**$O(N)$ since we visit each node exactly once`no matter the tree is balanced or not`

.**Space:**$O(N)$ in the worst case; $O(\log{N})$ in the best case (balanced tree).

- Tail Recursion + BFS
- The above recursion solution is probably not optimal in terms of
`space complexity`

. We can tweak the solution and make it`tail recursion`

, so the compiler might optimize our code (reusing the same function call stack frame). - To make it tail recursion, we just need to make sure that the recursive call is the
`last action`

in the function. **Note:**Optimization of tail recursion is not supported by Python or Java. And it is quite complicated.**Note:**A function cannot be tail recursion if there are`multiple occurrences`

of recursive calls in the function, even if the last action is the recursive call, e.g.`return f(n - 1) + f(n - 1)`

.

- The above recursion solution is probably not optimal in terms of
- Iteration
- With the help of a stack, we can simulate the function call process.
**Idea:**Keep the next nodes to visit in a stack**Time:**$O(N)$**Space:**$O(N)$ in the worst case; $O(\log{N})$ in the best case (balanced tree).

## Code

**Test Case:**

1 | [3,9,20,null,null,15,7] |

### Recursion

1 | public int maxDepth(TreeNode root) { |

### Iteration

**Note:**

- The initial depth should be
`1`

.

1 | public int iteration(TreeNode root) { |