LeetCode 103. Binary Tree Zigzag Level Order Traversal-优快云博客

本文链接：https://blog.youkuaiyun.com/u011327333/article/details/78846605

本文介绍了一种特殊的二叉树遍历方式——Z字型层序遍历，并提供了一段C++实现代码。该遍历方式会先从左到右遍历一层节点，再从右到左遍历下一层节点，以此类推。

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103. Binary Tree Zigzag Level Order Traversal

Description

Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its zigzag level order traversal as:

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

思路

Z 字型层序遍历二叉树，和普通的层序遍历差不多。只是在存储和输出每层的数据时要注意一下。

Code

/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        if (root == NULL) return {};
        deque<TreeNode*> dq;
        vector<vector<int>> ret;
        dq.push_back(root);
        int level = 0;
        while( !dq.empty() ) {
            int cur_size = dq.size();
            vector<int> tmp;
            std::reverse(dq.begin(), dq.end());
            if (level % 2 == 0) {
                for (int i = 0; i < cur_size; i++) {
                    TreeNode* node = dq.front();
                    tmp.push_back(node->val);
                    dq.pop_front();
                    if (node->left) dq.push_back(node->left);
                    if (node->right) dq.push_back(node->right);
                }
            }
            else {
                for (int i = 0; i < cur_size; i++) {
                    TreeNode * node = dq.front();
                    tmp.push_back(node->val);
                    dq.pop_front();
                    if (node->right) dq.push_back(node->right);
                    if (node->left) dq.push_back(node->left);
                }
            }
            ret.push_back(tmp);
            level++;
        }
        return ret;
    }
};