leetcode 103. Binary Tree Zigzag Level Order Traversal

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#leetcode #103 #Binary Tree Zigzag L

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本文介绍了一种特殊的二叉树遍历方式——锯齿形层级遍历，通过广度优先搜索（BFS）算法实现，对二叉树节点值按层级从左至右再从右至左交替输出。

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]
]

类似图的层级遍历，一定用BFS

/**
 * 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;
        queue<TreeNode *> que;
        if (root)
            que.push(root);
        
        while (!que.empty())
        {
            int size = que.size();
            vector<int> temp;
            for (int i = 0; i < size; i++)
            {
                TreeNode * p = que.front();
                que.pop();
                temp.push_back(p->val);
                if (p->left)
                    que.push(p->left);
                if (p->right)
                    que.push(p->right);
            }
            ret.push_back(temp);
        }
        for (int i = 1; i < ret.size(); i = i + 2)
            reverse(ret[i].begin(), ret[i].end());
        return ret;     
    }
};