LEETCODE: Binary Tree Level Order Traversal II-优快云博客

本文链接：https://blog.youkuaiyun.com/anyicheng2015/article/details/42294607

本文介绍如何使用队列实现二叉树的层次遍历，并通过实例展示从叶子节点到根节点的反向层次遍历顺序。

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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:

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

我估计下面这个方法不能算是最优的，大牛肯定有更简单的方法。

/**
 * Definition for binary tree
 * 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> > results;
        queue<TreeNode *> onelevel;
        if(root == NULL)    
            return results;
        onelevel.push(root);
        while(!onelevel.empty()) {
            vector<int> onelevelvals;
            queue<TreeNode *> newlevel;
            while(!onelevel.empty()) {
                TreeNode *front = onelevel.front();
                onelevelvals.push_back(front->val);
                if(front->left != NULL)
                    newlevel.push(front->left);
                if(front->right != NULL)
                    newlevel.push(front->right);
                onelevel.pop();
            }
            results.push_back(onelevelvals);
            onelevel = newlevel;
        }
        reverse(results.begin(), results.end());
        return results;
    }
};