LeetCode 103. Binary Tree Zigzag Level Order Traversal

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]
]

思路

记住上一行的输出（结果）节点，每一次从后往前读出下一行，这样第2,4,6次读的时候其实就会翻转成正向了。唯一的问题是左右子节点写入下一行的顺序，按照当前顺序区分一下就行。不是左右左右，就是右左右左，具体看code吧。

代码

/**
 * 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;
        vector<TreeNode*>* currRow = new vector<TreeNode*>();
        vector<TreeNode*>* nextRow = new vector<TreeNode*>();
        bool direction = true;  //true: R,L,R,L,...; false:L,R,L,R,...
        currRow->push_back(root);
        while(!currRow->empty()){
            //Output current row
            vector<int> currRowResult;
            for(int i=0; i<currRow->size(); i++){
                currRowResult.push_back( (*currRow)[i]->val );
            }
            result.push_back(currRowResult);
            //
            for(int i=currRow->size()-1; i>=0; i--){
                TreeNode* currNode = (*currRow)[i];
                if(direction){
                    if(currNode->right != NULL)
                        nextRow->push_back(currNode->right);
                    if(currNode->left != NULL)
                        nextRow->push_back(currNode->left);
                } else {
                    if(currNode->left != NULL)
                        nextRow->push_back(currNode->left);
                    if(currNode->right != NULL)
                        nextRow->push_back(currNode->right);
                }
            }
            //swap currRow and nextRow
            vector<TreeNode*>* temp = currRow;
            currRow = nextRow;
            nextRow = temp;
            nextRow->clear();
            direction = !direction;
        }
        return result;
    }
};