LeetCode:Binary Tree Level Order Traversal II (按层遍历)

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [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) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> result;
        traversal(root,1,result);
        reverse(result.begin(),result.end()); //在原先的题的基础上reverse一下即可
        return result;
        
        
    }
    //先序遍历 压入结果
    void traversal(TreeNode *node,int level,vector<vector<int>> &result)
    {  
        if(node==NULL) return;
        
        if(level>result.size())
            result.push_back(vector<int>());
        result[level-1].push_back(node->val);
        traversal(node->left,level+1,result);
        traversal(node->right,level+1,result);
        
    }
};