103_zigzagLevelOrder.txt

/**
 * 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) {
        if(root == NULL)
            return {};
        vector<vector<int>> res;
        queue<TreeNode*> helper;//辅助队列每次只存放该层的所有元素
        TreeNode* p;
        int flag = 0;
        helper.push(root);//将根节点入队
        while(helper.empty()==0){//队列不空
            deque<int> temp;//用双端队列来控制锯齿
            int size = helper.size();
            for(int i = 0;i < size;i++){//将本层的元素全部加入到temp中并把下一层元素加入队列中
                p = helper.front();
                if(flag%2==0){
                    temp.push_back(p->val);   
                }
                else{
                    temp.push_front(p->val);   
                }
                helper.pop();
                if(p->left)
                    helper.push(p->left);
                if(p->right)
                    helper.push(p->right);
            }
            res.push_back(vector<int>(temp.begin(),temp.end()));
            flag++;
        }
        return res;
    }
};