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   7return its bottom-up level order traversal as:[  [15,7],  [9,20],  [3]]Subscribe to see which companies asked this question.

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);        }   }};