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    /   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.

Analyse:

1. Recursion:if the level does isn't exist, create it and then push corresponding value into it.

Runtime:4ms.

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  */Ten classSolution { One  Public: Avector<vector<int>> Levelorder (treenode*root) { -vector<vector<int> >result; -Traverse (Root,0, result); the         returnresult; -     } -     voidTraverse (treenode* Root,intLevel, vector<vector<int> >&result) { -         if(!root)return; +         if(Level = = Result.size ())//The level does not exist and need to create it -Result.push_back (vector<int> ()); +          AResult[level].push_back (root->val); atTraverse (Root->left, Level +1, result); -Traverse (Root->right, Level +1, result); -     } -};

2. iteration:using queue to store nodes. When poping the first node, we need to add their children (child) to the queue. Then keep poping until the queue is empty.

Runtime:8ms.

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  */Ten classSolution { One  Public: Avector<vector<int> > Levelorder (treenode*root) { -vector<vector<int> >result; -         if(!root)returnresult; the          -queue<treenode* >Qu; - Qu.push (root); -          while(!Qu.empty ()) { +             intn =qu.size (); -vector<int> level;//Store the visited nodes in +              while(n--){ Atreenode* temp = Qu.front ();//Pop The first node in the queue atLevel.push_back (temp->val); - Qu.pop (); -                 if(temp->left) Qu.push (temp->left);//Add it Children (child) -                 if(temp->right) Qu.push (temp->Right ); -             } - result.push_back (level); in         } -         returnresult; to     } +};

Binary Tree level Order traversal