[LeetCode]Binary Tree Level Order Traversal

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.

 

BFS，广度优先搜索。递归实现。迭代版需要用到队列。

/**
 * 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:
    void bfs(TreeNode* root,int level,vector<vector<int>>& result)
    {
        if(!root) return;
        if(level>result.size()) result.push_back(vector<int>());
        result[level-1].push_back(root->val);
        bfs(root->left,level+1,result);
        bfs(root->right,level+1,result);
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        bfs(root,1,result);
        return result;
    }
};