leetcode 102 Binary Tree Level Order Traversal（难易度：Easy）

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).

For example:

Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its level order traversal as:

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

confused what

"{1,#,2,3}"

means?
> read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

1
/ \
2   3
/
4
\
5

The above binary tree is serialized as

"{1,2,3,#,#,4,#,#,5}"

.

代码：

//解法一：（递归）
<pre name="code" class="cpp">/**

* 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) {traverse(root,
1, result);return result;}void traverse(struct TreeNode *root, size_t level, vector<vector<int>> &result) {if (root == NULL)return;if (level > result.size())result.push_back(vector<int>());result[level - 1].push_back(root->val);traverse(root->left, level +
1, result);traverse(root->right, level + 1, result);}private:vector<vector<int>> result;};

//解法二：（迭代）
/**
* 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 == nullptr)
return result;
queue<TreeNode *> current, //当前层的节点
next; //下一层的节点
vector<int> level;

current.push(root);
while (!current.empty()) {
while (!current.empty()) {
TreeNode *node = current.front();
current.pop();
level.push_back(node->val);
if (node->left != nullptr) next.push(node->left);
if (node->right != nullptr) next.push(node->right);
} //当前层节点全部处理完
result.push_back(level);
level.clear();
swap(next, current);
}
return result;
}
private:
vector<vector<int>> result;
};

原题地址：https://leetcode.com/problems/binary-tree-level-order-traversal/