按層輸出二叉樹，廣度優先。

    3
   /   9  20
    /     15   7     

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

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
//廣度優先遍歷
class Solution {
 
public:
    vector<vector<int>> rres;
    
    void levelTree(TreeNode*root, int level){
        if (root == NULL)
            return;
        if (level == rres.size()){
            vector<int> res;
            rres.push_back(res);
        }
        
        rres[level].push_back(root
 
->val);
        levelTree(root->left,level+1);
        levelTree(root->right,level+1);
    }
    
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        levelTree(root,0);
        //return reverse(rres.begin(),rres.end());
        return vector<vector<int> >(rres.rbegin(), rres.rend());
    }
};
 

【easy】107. Binary Tree Level Order Traversal II 按層輸出二叉樹