LeetCode-103-Binary Tree Zigzag Level Order Traversal

发布时间：2020-12-14 04:26:10 所属栏目：大数据来源：网络整理

导读：算法描述： 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,15,7] , 3 / 9

算法描述：

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,15,7],

    3
   /   9  20
    /     15   7

return its zigzag level order traversal as:

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

解题思路：与水平访问一样，用队列实现。区别在于加入一个判断，在偶数行反向插入数值就行。

    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> results;
        helper(root,results,0);
        return results;
    }
    
    void helper(TreeNode* root,vector<vector<int>>& results,int level){
        if(root ==nullptr) return;
        if(results.size() <= level){
            vector<int> temp;
            results.push_back(temp);
        }
        if(level%2==0){
            results[level].push_back(root->val);
        }else{
            results[level].insert(results[level].begin(),root->val);
        }

        helper(root->left,level+1);
        helper(root->right,level+1);

    }

（编辑：李大同）

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