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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
int maxDepth(struct TreeNode* root) {
    if(root == NULL)
        return 0;
    int x = maxDepth(root->left);
    int y = maxDepth(root->right);
    if(x>y)
        return x+1;
    else return y+1;
}