給定一個二叉樹，找出其最大深度。

二叉樹的深度為根節點到最遠葉子節點的最長路徑上的節點數。

說明:葉子節點是指沒有子節點的節點。

方法一：遞迴

====================Python=========================

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        
 
if not root:
            return 0
        return max(self.maxDepth(root.left),self.maxDepth(root.right)) + 1

====================Java====================

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

====================Golang=========================

func maxDepth(root *TreeNode) int {
    
 
if root == nil {
        return 0
    }
    return max(maxDepth(root.Left), maxDepth(root.Right)) + 1
}
func max(a, b int) int{
    if a > b {
        return a
    } else {
        return b
    }
}

====================PHP==============================

class Solution {

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        if (!$root) {
            return 0;
        }
        return max($this->maxDepth($root->left), $this->maxDepth($root->right)) + 1;
    }
}

方法二：BFS

====================Python====================

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        from collections import deque
        q = deque()
        q.append(root)
        q_help = deque()
        res = 0
        while q_help or q:
            while q:
                x = q.popleft()
                if x.left:
                    q_help.append(x.left)
                if x.right:
                    q_help.append(x.right)
            res += 1
            q = q_help
            q_help = deque()
        return res

====================Java====================

class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        } 
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        int ans = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            while (size > 0) {
                TreeNode node = queue.poll();
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
                size--;
            }
            ans++;
        }
        return ans;
    }
}

====================Golang====================

func maxDepth(root *TreeNode) int {
    if root == nil {
        return 0
    }
    queue := []*TreeNode{}
    queue = append(queue, root)
    ans := 0
    for len(queue) > 0 {
        sz := len(queue)
        for sz > 0 {
            node := queue[0]
            queue = queue[1:]
            if node.Left != nil {
                queue = append(queue, node.Left)
            }
            if node.Right != nil {
                queue = append(queue, node.Right)
            }
            sz--
        }
        ans++
    }
    return ans
}