BFS

/**
 * 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 treeDepth(TreeNode* root) {
        if (!root) return 0;
        // bfs
        queue<TreeNode*> q;
        q.push(root);
        int cnt = 0;
        while (!q.empty()) {
            int count = q.size(); // 当前层大小
            while (count--) {
                auto t = q.front();
                q.pop();
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
            }
            cnt++;
        }

        return cnt;
    }
};