二叉树的之字形层序遍历

C++ BFS 改造，加一个翻转

因为是之字形，所以偶数行flag=1是从左到右遍历，奇数行是flag=0，逆序遍历。

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > zigzagLevelOrder(TreeNode* root) {
        vector<vector<int> >res;
        queue<TreeNode*> que;
        if(root==nullptr) return res;
        que.push(root);
        int flag = 1;
        while(!que.empty()){
            int size = que.size();
            vector<int> temp;
            for(int i=0;i<size;i++){
                TreeNode* node = que.front();
                que.pop();
                if(node->left) que.push(node->left);
                if(node->right) que.push(node->right);
                if(flag){
                    temp.push_back(node->val);
                }else{
                    temp.insert(temp.begin(), node->val);
                }
            }
            flag = 1-flag;
            res.push_back(temp);
        }
        return res;
    }
};