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]
]


在102的基础上稍加改动即可

class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        
       queue<TreeNode*> q;
       queue<int> q2;
       vector<vector<int> > ans;
       if(root==NULL)
           return ans;
       q.push(root);q2.push(0);
       vector<int> res;
       int j=-1;
       int tag = 0;
       while(!q.empty())
       {
           TreeNode* term = q.front();
           int i = q2.front();
           if(i==j+2)
           {
               if(tag==1)
               {
                   vector<int> a;
                   for(int i=res.size()-1;i>=0;i--)
                   {
                       a.push_back(res[i]);
                   }
                   tag=0;
                   ans.push_back(a);
               }
               else
               {
                   ans.push_back(res);
                   tag=1;
               }
               res.clear();
               j++;
           }
           res.push_back(term->val);     
           q.pop();q2.pop();
           if(term->left!=NULL)
           { q.push(term->left);q2.push(i+1);}
           if(term->right!=NULL)
           { q.push(term->right);q2.push(i+1);}
       }
       if(!res.empty())
       {
           
               if(tag==1)
               {
                   vector<int> a;
                   for(int i=res.size()-1;i>=0;i--)
                   {
                       a.push_back(res[i]);
                   }
                   tag=0;
                   ans.push_back(a);
               }
               else
                   ans.push_back(res);
            
       }
           
        
       return ans;      
        
    }
};