迭代

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

class Solution(object):
    def maxDepth(self, root):
        if root is None:
            return 0
        de=0
        qu=collections.deque()
        qu.append(root)
        while qu:
            le=len(qu)
            while le>0:
                tn=qu.popleft()
                if tn.left:
                    qu.append(tn.left)
                if tn.right:
                    qu.append(tn.right)
                le-=1
            de+=1
        return de

递归

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

class Solution(object):
    def maxDepth(self, root):
        de=0
        nod=root
        def Explore(nod,de):
            if nod is None:
                return de
            de+=1
            return max(Explore(nod.left,de),Explore(nod.right,de))
        return Explore(root,de)