技術標籤：leetcode每日一題202012leetcode廣度優先搜尋

Problem

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:

Solution

/**
 * 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) {
 

        vector<vector<int>> ret;
        if(!root)
            return ret;
        
        vector<int> level;
        int flag = 0;

        queue<TreeNode *> tn_queue;
        tn_queue.push(root);

        while(!tn_queue.empty())
        {
            int cur_level_size = tn_queue.
 
size();
            for(int i = 0;i<cur_level_size;++i)
            {
                TreeNode * tmp = tn_queue.front();
                tn_queue.pop();
                level.push_back(tmp->val);

                if(tmp->left)
                    tn_queue.push(tmp->left);
                if(tmp->right)
                    tn_queue.push(tmp->right);
            }
            if(flag == 0)
            {
                flag = 1;
            }
            else
            {
                reverse(level.begin(),level.end());
                flag = 0;
            }
            ret.push_back(level);
            level.clear();
        }

        return ret;
        
    }
};