LeetCode – Refresh – Binary Tree Level Order Traversal ii-优快云博客

本文介绍了一种使用栈和队列实现二叉树层次遍历并逆序输出的方法。通过定义一个特殊的数据结构存储每层节点，再反转顺序输出，实现了从底层到顶层的层次遍历。

This question is almost same as the previous one.

I just use a stack (another vector) of vector<int> to store the level result. Then reverse it.

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int> > levelOrderBottom(TreeNode *root) {
13         vector<vector<int> > result;
14         if (!root) return result;
15         stack<vector<int> > s;
16         queue<TreeNode *> q;
17         vector<int> level;
18         q.push(root);
19         int current = 1, future = 0;
20         while (!q.empty()) {
21             TreeNode *tmp = q.front();
22             q.pop(), current--;
23             if (tmp->left) {
24                 q.push(tmp->left);
25                 future++;
26             }
27             if (tmp->right) {
28                 q.push(tmp->right);
29                 future++;
30             }
31             level.push_back(tmp->val);
32             if (!current) {
33                 current = future;
34                 future = 0;
35                 s.push(level);
36                 level.clear();
37             }
38         }
39         while (!s.empty()) {
40             result.push_back(s.top());
41             s.pop();
42         }
43         return result;
44     }
45 };