leetcode 103二叉树的锯齿形层次遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
/**
最直接的想法是利用层次遍历，然后reverse偶数层的数据

**/
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        if(root==NULL) return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        TreeNode* p;
        q.push(root);
        int flag=1;
        while(!q.empty()){
            int k=q.size();
            vector<int> level;
            for(int i=0;i<k;i++){
                p=q.front();
                q.pop();
                level.push_back(p->val);
                if(p->left) q.push(p->left);
                if(p->right) q.push(p->right);
            }
            if(flag==0) reverse(level.begin(),level.end());
            flag=(flag+1)%2;
            res.push_back(level);
        }
        return res;
    }
};