解析

//!  n,n->时间上每个元素入队出队一次,共2n次;空间上如果为完全二叉树，队列最长n/2
class Solution {
 public:
  int maxDepth(TreeNode *root) {
    if (root == nullptr) {
      return 0;
    }
    TreeNode *node = root;
    int res = 0;
    queue<TreeNode *> treeQueue;
    treeQueue.push(node);
    while (!treeQueue.empty()) {
      res += 1;
      int levelSize = treeQueue.size();
      for (int i = 0; i < levelSize; ++i) {
        node = treeQueue.front();
        treeQueue.pop();
        if (node->left != nullptr) {
          treeQueue.push(node->left);
        }
        if (node->right != nullptr) {
          treeQueue.push(node->right);
        }
      }
    }
    return res;
  }
};