102.binary-tree-level-order-traversal: add C++ implementation (azl397985856#22)

Author: raof01

problems/102.binary-tree-level-order-traversal.md

@@ -30,6 +30,10 @@ return its level order traversal as:
 
 然后不断的出队， 如果出队的是null，则表式这一层已经结束了，我们就继续push一个null。
 
+如果不入队特殊元素Null来表示每层的结束，则在while循环开始时保存当前队列的长度，以保证每次只遍历一层（参考下面的C++ Code）。
+
+> 如果采用递归方式，则需要将当前节点，当前所在的level以及结果数组传递给递归函数。在递归函数中，取出节点的值，添加到level参数对应结果数组元素中（参考下面的C++ Code）。
+
 
 ## 关键点解析
 
@@ -43,7 +47,9 @@ return its level order traversal as:
 
 
 ## 代码
+* 语言支持：JS，C++
 
+Javascript Code:
 ```js
 /*
  * @lc app=leetcode id=102 lang=javascript
@@ -123,7 +129,66 @@ var levelOrder = function(root) {
 
   return items;
 };
+
+```
+C++ Code:
+```C++
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+ 
+ // 迭代
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        auto ret = vector<vector<int>>();
+        if (root == nullptr) return ret;
+        auto q = vector<TreeNode*>();
+        q.push_back(root);
+        auto level = 0;
+        while (!q.empty())
+        {
+            auto sz = q.size();
+            ret.push_back(vector<int>());
+            for (auto i = 0; i < sz; ++i)
+            {
+                auto t = q.front();
+                q.erase(q.begin());
+                ret[level].push_back(t->val);
+                if (t->left != nullptr) q.push_back(t->left);
+                if (t->right != nullptr) q.push_back(t->right);
+            }
+            ++level;
+        }
+        return ret;
+    }
+};
+
+// 递归
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        vector<vector<int>> v;
+        levelOrder(root, 0, v);
+        return v;
+    }
+private:
+    void levelOrder(TreeNode* root, int level, vector<vector<int>>& v) {
+        if (root == NULL) return;
+        if (v.size() < level + 1) v.resize(level + 1);
+        v[level].push_back(root->val);
+        levelOrder(root->left, level + 1, v);
+        levelOrder(root->right, level + 1, v);
+    }
+};
 ```
+
 ## 相关题目
 - [103.binary-tree-zigzag-level-order-traversal](./103.binary-tree-zigzag-level-order-traversal.md)
 - [104.maximum-depth-of-binary-tree](./104.maximum-depth-of-binary-tree.md)