题解 | #从上往下打印二叉树#

/*
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};*/
#include <cstdio>
#include <queue>
#include <vector>
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
		vector<int> res;
		if (root == nullptr)
			return res;

		queue<TreeNode*> queue;
		queue.push(root);

		int nextLevel = 0;
		int toBePrinted = 1;

		while (!queue.empty()) {
			TreeNode *pTreeNode = queue.front();
			res.push_back(pTreeNode->val);
			
			queue.pop();
			toBePrinted--;

			if (pTreeNode->left != nullptr) {
				queue.push(pTreeNode->left);
				nextLevel++;
			}
			if (pTreeNode->right != nullptr) {
				queue.push(pTreeNode->right);
				nextLevel++;
			}

			if (toBePrinted == 0) {
				printf("\n");
				toBePrinted = nextLevel;
				nextLevel = 0;
			}
		}
		return res;
    }
};