dfs记录当前层，遇到叶子更新全局最大层数

class Solution {
public:

    int ans=1;
    int treeDepth(TreeNode* root) {
        if(!root) return 0;
        dfs(root,0);
        return ans; 
    }

    void dfs(TreeNode* r,int lev){
        if(!r){
            ans=max(lev,ans);
            return;
        } 

        dfs(r->left,lev+1);//写成lev++,需要恢复现场
        dfs(r->right,lev+1);
    }
};

bfs 每次都把上一层弹出

class Solution {
public:

    int treeDepth(TreeNode* root){
        if(!root) return 0;


        queue<TreeNode*> q;
        q.push(root);

        int lev=0;

        while(!q.empty()){
            TreeNode* tail=q.back(),*head;//记录上一层的末尾

            while(!q.empty()){
                head=q.front();q.pop();
                if(head->left) q.push(head->left);
                if(head->right) q.push(head->right);

                if(head==tail) break;//此时上一层都已经出队
            }

            lev++;

        }

        return lev;
    }
};