leetcode日记（90）二叉树的锯齿形层序遍历

和上一题思路差不多，只不过有一个陷阱，就是不能用上一题的递归解法，因为逆序遍历会导致后面需要顺序的节点也逆序。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> result;
    vector<TreeNode*> tree;
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        if(root==NULL) return result;
        tree.push_back(root);
        while(!tree.empty()){
            int sum=tree.size();
            result.push_back(vector<int> ());
            for(int i=0;i<sum;i++){
                if(tree[i]->left) tree.push_back(tree[i]->left);
                if(tree[i]->right) tree.push_back(tree[i]->right);
            }
            if(result.size()%2!=1) for(int i=sum-1;i>=0;i--) result.back().push_back(tree[i]->val);
            else for(int i=0;i<sum;i++) result.back().push_back(tree[i]->val);
            tree.erase(tree.begin(),tree.begin()+sum);
        }
        return result;
    }
};