103. Binary Tree Zigzag Level Order Traversal

清疚 2022-05-12 15:56 150阅读 0赞

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]`,

3
       / \
      9  20
        /  \
       15   7

return its zigzag level order traversal as:

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

/**
     * 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>> result;
             if(root==NULL)
                return result;
    
            deque<TreeNode*> nodes;
            nodes.push_back(root);
            int flag = 1;
            while(!nodes.empty())
            {
                vector<int> res;
                int len = nodes.size();
                while(len>0)
                {
                    if(flag == 1)
                    {
                        root = nodes.front();
                        nodes.pop_front();
                        res.push_back(root->val);
    
                        if(root->left)
                            nodes.push_back(root->left);
                        if(root->right)
                            nodes.push_back(root->right);
                    }
                    else
                    {
                        root = nodes.back();
                        nodes.pop_back();
                        res.push_back(root->val);
                        if(root->right)
                            nodes.push_front(root->right);
                        if(root->left)
                        nodes.push_front(root->left);
                    }               
                    len--;               
                } 
               result.push_back(res); 
               flag *=-1;
            }
            return result;
        }
    };