LeetCode 0103 - Binary Tree Zigzag Level Order Traversal

Binary Tree Zigzag Level Order Traversal

Desicription

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],

代码语言：javascript复制

  3
 / 
9  20
  /  
 15   7

return its zigzag level order traversal as:

代码语言：javascript复制

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

Solution

代码语言：javascript复制

/**
 * 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 {
private:
    vector<vector<int>> res;
    void buildVec(TreeNode* root, int level) {
        if(root == NULL)
            return ;
        if(level == res.size())
            res.push_back(vector<int>());
        res[level].push_back(root->val);
            buildVec(root->left, level 1);
            buildVec(root->right, level 1);
    }
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        buildVec(root, 0);
        for(int i = 0; i < res.size(); i  ) {
            if(i & 1) {
                for(int left = 0, right = res[i].size() - 1; left <= right; left  , right--) {
                    swap(res[i][left], res[i][right]);
                }
            }
        }
        return res;
    }
};

binary null return traversal tree

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