Leetcode——107. Binary Tree Level Order Traversal II

题目

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,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its bottom-up level order traversal as:
[
  [15,7],
  [9,20],
  [3]
]
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解答

借鉴先序遍历，注意添加层号。

/**
 * 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) {
        int high=height(root);
        vector<vector<int>> res(high,vector<int>());
        levelOrder(root,0,res);
        return res;
    }
private:
    int height(TreeNode* root)
    {
        if(root==NULL)
            return 0;
        else
            return max(height(root->left),height(root->right))+1;
    }
   void levelOrder(TreeNode* root,int level,vector<vector<int>> &res) 
   {
       if(root==NULL)
            return;
       else
        {
            res[res.size()-1-level].push_back(root->val);
            levelOrder(root->left,level+1,res);
            levelOrder(root->right,level+1,res);
        }
   }
};