【leetcode】Binary Tree Level Order Traversal II

二叉树层次遍历

最新推荐文章于 2025-11-27 12:38:18 发布

最新推荐文章于 2025-11-27 12:38:18 发布 · 57 阅读

文章标签：

#c/c++

本文介绍了如何使用C++实现二叉树的层次遍历，并从叶子节点到根节点进行输出。通过两种不同的方法实现这一功能，一种是利用队列记录每一层的节点数，另一种则是使用两个队列交替进行节点的层次遍历。

Question ：

Given a binary tree, return thebottom-up level ordertraversal 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],
]

confused what"{1,#,2,3}"means?> read more on how binary tree is serialized on OJ.

Anwser 1 ：

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int>> ret;
        
        if(root == NULL) return ret;
        
        vector<int> vec;
        queue<TreeNode *> Q;
        
        Q.push(root);
        int count = 1;
        
        while(!Q.empty()){
            
            vec.clear();
            int nextCount = 0;      // cal next row count
            for(int i = 0; i < count; i++){     // one row count
                TreeNode *tmp = Q.front();
                Q.pop();
                
                vec.push_back(tmp->val);        // save one row val
                if(tmp->left){
                    Q.push(tmp->left);
                    nextCount++;
                }
                if(tmp->right){
                    Q.push(tmp->right);
                    nextCount++;
                }
            }
            count = nextCount;
            ret.push_back(vec);
        }
        
        reverse(ret.begin(), ret.end());        //reverse vector
        return ret;
    }
};

注意点：添加了一行 reverse(ret.begin(), ret.end()); // reverse vector

Anwser 2 ：

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int>> ret;
        if(root == NULL) return ret;
        
        vector<int> vec;
        queue<TreeNode *> Q;
        queue<TreeNode *> Q2;   // extra space
        
        Q.push(root);
        while(!Q.empty()){
            TreeNode *tmp = Q.front();
            Q.pop();
            
            if(tmp != NULL){
                vec.push_back(tmp->val);
                if(tmp->left) Q2.push(tmp->left);
                if(tmp->right) Q2.push(tmp->right);
            }
            
            if(Q.empty()){      // one row end
                ret.push_back(vec);
                vec.clear();
                swap(Q, Q2);
            }
        }
        
        reverse(ret.begin(), ret.end());    // reverse vector
    }
};

注意点：添加了一行 reverse(ret.begin(), ret.end()); // reverse vector

参考推荐：

Binary Tree Level Order Traversal