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]
]
用lev记录当前树到了第几层。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
vector<vector<int> > re;
    vector<vector<int> > levelOrder(TreeNode *root) {
        if(root == NULL)return re;
        level(root,1);
        return re;
    }
    void level(TreeNode *root, int lev)
    {
        if(root == NULL)return;
        if(lev > re.size())
        {
            vector<int > temp;
            re.push_back(temp);
        }
        re[lev-1].push_back(root->val);
        level(root->left,lev+1);
        level(root->right,lev+1);
    }
};