可以利用層次遍歷，記錄層數即二叉樹的深度（注意統計每一層結點的個數，以免影響記錄層數）

//藉助層次遍歷，非遞迴
class Solution {
public:
    int TreeDepth(TreeNode* pRoot)
    {
        queue<TreeNode*> q;
        if (!pRoot) return 0;
        q.push(pRoot);
        int level = 0;
        while (!q.empty()) {
            int len = q.size();
            level++;
            while
 
 (len--) {
                TreeNode* tem = q.front();
                q.pop();
                if (tem->left) q.push(tem->left);
                if (tem->right) q.push(tem->right);
            }
        }
        return level;
    }
};

//遞迴
class Solution {
public:
    int TreeDepth(TreeNode* pRoot)
    {
        if
 
 (pRoot == NULL)
            return 0;
        else
            return max(1 + TreeDepth(pRoot->left), 1 + TreeDepth(pRoot->right));

    }
};