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Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

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/** * 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 maxDepth(TreeNode* root) {        int currentDepth = 0;        maxDep = 0;        if(root != NULL)        {            travelBinaryTree(root, currentDepth);        }        return maxDep;    }private:    int maxDep;    void travelBinaryTree(TreeNode* node, int depth)    {        depth++;        maxDep = (depth > maxDep) ? depth : maxDep;        if(node->left)            travelBinaryTree(node->left, depth);        if(node->right)            travelBinaryTree(node->right, depth);    }};

class Solution {public:    int maxDepth(TreeNode* root) {        if (!root)            return 0;        return 1 + max(maxDepth(root->right), maxDepth(root->left));    }};