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,#,#,15,7},

return its bottom-up level order traversal as:

分析

与 LeetCode（103） Binary Tree Zigzag Level Order Traversal 以及 LeetCode（102） Binary Tree Level Order Traversal 本质相同的题目，只不过灵活调整结果返回格式罢了。

AC代码

/** * 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) {        //层次遍历，分层存储        if (!root)            return vector<vector<int> >();        vector<vector<int> > ret;        //定义两个队列，一个存储所有的父节点,另一个存储他们的子节点也就是子层        queue<TreeNode *> parents;        parents.push(root);        while (!parents.empty())        {            //存储当前层的遍历结果            vector<int> tmp;            //定义队列存储他们的子节点也就是子层            queue<TreeNode *> childs;            while (!parents.empty())            {                TreeNode *node = parents.front();                tmp.push_back(node->val);                //弹出当前父节点                parents.pop();                if (node->left)                    childs.push(node->left);                if (node->right)                    childs.push(node->right);            }            //存储当前层的遍历结果            ret.push_back(tmp);            //遍历下一层            parents = childs;        }        //反转遍历结果 由下向上存储        reverse(ret.begin(), ret.end());        return ret;    }};

GitHub测试程序源码