python二叉树的最大深度该怎样理解

/**
 * 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 maxDept = 0;
    int dept = 1;
    int maxDepth(TreeNode* root) {
        if(root == NULL){
            return 0;
        }else{
            int leftHeight = maxDepth(root->left);
            int rightHeight = maxDepth(root->right);
            return leftHeight>rightHeight ? leftHeight + 1 :  rightHeight + 1;
        }
    }
};