分行从上往下打印二叉树.md

题目

从上到下按层打印二叉树，同一层的结点按从左到右的顺序打印，每一层打印到一行。

样例

输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null]
    8
   / \
  12  2
     /
    6
   /
  4

输出：[[8], [12, 2], [6], [4]]

参考答案

/**
 * 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:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        q.push(NULL); //root层的标识符

        vector<int> cur;
        while(q.size()){
            TreeNode* t = q.front();
            q.pop();

            if(t){ //跟上一道题同样的操作
                cur.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            else{
                if(q.size()) q.push(NULL); 
                res.push_back(cur); 
                cur.clear();
            }
        }
        return res;

    }
};