算法1

思路：递归遍历左右子树，求出最大值+1

C++ 代码

/**
 * 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 treeDepth(TreeNode* root) {
        if(!root)return 0;
        return max(treeDepth(root->left),treeDepth(root->right))+1;
    }
};