之字形打印二叉树.md

题目

请实现一个函数按照之字形顺序从上向下打印二叉树。

即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，第三行再按照从左到右的顺序打印，其他行以此类推。

样例

输入如下图所示二叉树[8, 12, 2, null, null, 6, 4, null, null, null, null]
    8
   / \
  12  2
     / \
    6   4
输出：[[8], [2, 12], [6, 4]]

参考答案

/**
 * 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<int> get_val(vector<TreeNode*> level)
    {
        vector<int> res;
        for (auto &u : level)
            res.push_back(u->val);
        return res;
    }

    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>>res;
        if (!root) return res;
        vector<TreeNode*>level;
        level.push_back(root);
        res.push_back(get_val(level));
        bool zigzag = true;
        while (true)
        {
            vector<TreeNode*> newLevel;
            for (auto &u : level)
            {
                if (u->left) newLevel.push_back(u->left);
                if (u->right) newLevel.push_back(u->right);
            }
            if (newLevel.size())
            {
                vector<int>temp = get_val(newLevel);
                if (zigzag)
                    reverse(temp.begin(), temp.end());
                res.push_back(temp);
                level = newLevel;
            }
            else break;
            zigzag = !zigzag;
        }
        return res;
    }
};