# 104 Maximum Depth of Binary Tree

## 104. Maximum Depth of Binary Tree

## 1. Question

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.

## 2. Implementation

**(1) DFS**

```java
class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}
```

**(2) BFS**

```java
class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int size = 0, depth = 0;

        while (!queue.isEmpty()) {
            size = queue.size();
            ++depth;

            for (int i = 0; i < size; i++) {
                TreeNode curNode = queue.remove();

                if (curNode.left != null) {
                    queue.add(curNode.left);
                }

                if (curNode.right != null) {
                    queue.add(curNode.right);
                }
            }
        } 
        return depth;
    }
}
```

## 3. Time & Space Complexity

DFS: 时间复杂度:O(n), 空间复杂度O(h)

BFS: 时间复杂度:O(n), 空间复杂度O(w), w为一层中有最多node的个数


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