LeetcCode102 Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). （Easy）

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

分析：

层序遍历，就是BFS的思路，利用队列把每一行的节点存进去，然后一行一行读出。

注意每次循环用读取size的方式确定这一行到哪里结束。

代码：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 private:
12     vector<vector<int>> result;
13 public:
14     vector<vector<int>> levelOrder(TreeNode* root) {
15         if (root == nullptr) {
16             return result;
17         }
18         queue<TreeNode* > que;
19         que.push(root);
20         while (!que.empty()) {
21             int sz = que.size();
22             vector<int> temp;
23             for (int i = 0; i < sz; ++i) {
24                 TreeNode* cur = que.front();
25                 que.pop();
26                 temp.push_back(cur -> val);
27                 if (cur -> left != nullptr) {
28                     que.push(cur -> left);
29                 }
30                 if (cur -> right != nullptr) {
31                     que.push(cur -> right);
32                 }
33             }
34             result.push_back(temp);
35         }
36         return result;
37     }
38 };