LeetCode 面试题32 - II. 从上到下打印二叉树 II

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

 

例如:
给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]
 

提示：

节点总数 <= 1000
注意：本题与主站 102 题相同：https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

 

/**
 * 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>> levelOrder(TreeNode* root) {
        if (root == NULL)
            return {};
        
        queue<TreeNode*> q;
        vector<vector<int>> res;
        q.push(root);
        while (!q.empty())
        {
            vector<int> level;
            int size = q.size();
            while (size--)
            {
                level.push_back(q.front()->val);
                if (q.front()->left)
                    q.push(q.front()->left);
                if (q.front()->right)
                    q.push(q.front()->right);
                q.pop();
            }
            res.push_back(level);
        }
        return res;
    }
};