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    1. /**
    2.  * Definition for a binary tree node.
    3.  * struct TreeNode {
    4.  *     int val;
    5.  *     TreeNode *left;
    6.  *     TreeNode *right; 
    7.  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
    8.  * }; 
    9.  */
    10. class Solution {
    11. public:
    12.     // int max = 0;
    13.     int treeDepth(TreeNode* root) {
    14.        if(root == NULL) return 0;
    15.        return max(treeDepth(root->left), treeDepth(root->right)) + 1;
    16.     }
    17. };