考察二叉树的遍历，使用递归形式实现。 自己写的基本和最优相同，只不过有些语句没有优化。test1.cpp为自己的，test2.cpp为参考改进过得

test1.cpp

/**
 * 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 depthLeft = 0,depthRight = 0;
        if(root == NULL) return 0;
        if(root->left == NULL && root->right == NULL)
            return 1;
        depthLeft = maxDepth(root->left);
        depthRight = maxDepth(root->right);
        
        return depthLeft >= depthRight ? depthLeft + 1 : depthRight + 1;
    }
};

test2.cpp

/**
 * 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) {
        return root == NULL ? 0: max(maxDepth(root -> left),maxDepth(root -> right)) + 1;
    }
};