树的高度

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int maxDepth(TreeNode* root) {
        if (root == nullptr)
            return 0;
        int submaxDepth = max(maxDepth(root -> left), maxDepth(root -> right));
        return submaxDepth + 1;
    }
};