题目描述 从上往下打印出二叉树的每个节点，同层节点从左至右打印。
思路：利用队列（queue）层次遍历二叉树
/* struct TreeNode { int val; struct TreeNode *left; struct TreeNode *right; TreeNode(int x) : val(x), left(NULL), right(NULL) { } };*/ class Solution { public: vector<int> PrintFromTopToBottom(TreeNode* root) { vector<int> res; if (!root) return res; queue <TreeNode*> treeNodeStack; treeNodeStack.push(root); while (!treeNodeStack.empty()) { TreeNode* currentNode = treeNodeStack.front(); treeNodeStack.pop(); if (currentNode->left) treeNodeStack.push(currentNode->left); if (currentNode->right) treeNodeStack.push(currentNode->right); res.push_back(currentNode->val); } return res; } };