1.递归遍历时通过“参数传递”自顶向下传递信息

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int ans = 0;//全局变量，在各级函数间共享信息
    int treeDepth(TreeNode* root) {
        dfs(root, 1);
        return ans;
    }

    void dfs(TreeNode* root, int depth){
        if(!root) return;
        ans = max(ans, depth);
        dfs(root->left, depth + 1);
        dfs(root->right, depth + 1);
    }
};

2.递归遍历时，通过“函数返回值”自底向上传递信息

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int ans = 0;
    int treeDepth(TreeNode* root) {
        return dfs(root);
    }

    int dfs(TreeNode* root){
        if(!root) return 0;
        int l = dfs(root->left);
        int r = dfs(root->right);

        return max(l, r) + 1;
    }
};